diff --git a/apps/web/src/app/(authenticated)/page.test.tsx b/apps/web/src/app/(authenticated)/page.test.tsx
index 3d72378..528ec61 100644
--- a/apps/web/src/app/(authenticated)/page.test.tsx
+++ b/apps/web/src/app/(authenticated)/page.test.tsx
@@ -1,112 +1,150 @@
-import { describe, it, expect, vi, beforeEach } from "vitest";
-import { render, screen, waitFor } from "@testing-library/react";
+import { describe, it, expect, vi, beforeEach, beforeAll } from "vitest";
+import { render, screen, waitFor, act } from "@testing-library/react";
 import DashboardPage from "./page";
-import { fetchDashboardSummary } from "@/lib/api/dashboard";
+import * as layoutsApi from "@/lib/api/layouts";
+import type { UserLayout, WidgetPlacement } from "@mosaic/shared";
 
-// Mock Phase 3 dashboard widgets
-vi.mock("@/components/dashboard/DashboardMetrics", () => ({
-  DashboardMetrics: (): React.JSX.Element => (
-    <div data-testid="dashboard-metrics">Dashboard Metrics</div>
+// ResizeObserver is not available in jsdom
+beforeAll((): void => {
+  global.ResizeObserver = vi.fn().mockImplementation(() => ({
+    observe: vi.fn(),
+    unobserve: vi.fn(),
+    disconnect: vi.fn(),
+  }));
+});
+
+// Mock WidgetGrid to avoid react-grid-layout dependency in tests
+vi.mock("@/components/widgets/WidgetGrid", () => ({
+  WidgetGrid: ({
+    layout,
+    isEditing,
+  }: {
+    layout: WidgetPlacement[];
+    isEditing?: boolean;
+  }): React.JSX.Element => (
+    <div data-testid="widget-grid" data-editing={isEditing}>
+      {layout.map((item) => (
+        <div key={item.i} data-testid={`widget-${item.i}`}>
+          {item.i}
+        </div>
+      ))}
+    </div>
   ),
 }));
 
-vi.mock("@/components/dashboard/OrchestratorSessions", () => ({
-  OrchestratorSessions: (): React.JSX.Element => (
-    <div data-testid="orchestrator-sessions">Orchestrator Sessions</div>
-  ),
-}));
-
-vi.mock("@/components/dashboard/QuickActions", () => ({
-  QuickActions: (): React.JSX.Element => <div data-testid="quick-actions">Quick Actions</div>,
-}));
-
-vi.mock("@/components/dashboard/ActivityFeed", () => ({
-  ActivityFeed: (): React.JSX.Element => <div data-testid="activity-feed">Activity Feed</div>,
-}));
-
-vi.mock("@/components/dashboard/TokenBudget", () => ({
-  TokenBudget: (): React.JSX.Element => <div data-testid="token-budget">Token Budget</div>,
-}));
-
-// Mock hooks and API calls
+// Mock hooks
 vi.mock("@/lib/hooks", () => ({
   useWorkspaceId: (): string | null => "ws-test-123",
 }));
 
-vi.mock("@/lib/api/dashboard", () => ({
-  fetchDashboardSummary: vi.fn().mockResolvedValue({
-    metrics: {
-      activeAgents: 5,
-      tasksCompleted: 42,
-      totalTasks: 100,
-      tasksInProgress: 10,
-      activeProjects: 3,
-      errorRate: 0.5,
-    },
-    recentActivity: [],
-    activeJobs: [],
-    tokenBudget: [],
-  }),
-}));
+// Mock layout API
+vi.mock("@/lib/api/layouts");
+
+const mockExistingLayout: UserLayout = {
+  id: "layout-1",
+  workspaceId: "ws-test-123",
+  userId: "user-1",
+  name: "Default",
+  isDefault: true,
+  layout: [
+    { i: "TasksWidget-default", x: 0, y: 0, w: 4, h: 2 },
+    { i: "CalendarWidget-default", x: 4, y: 0, w: 4, h: 2 },
+  ],
+  metadata: {},
+  createdAt: new Date("2026-01-01T00:00:00Z"),
+  updatedAt: new Date("2026-01-01T00:00:00Z"),
+};
 
 describe("DashboardPage", (): void => {
   beforeEach((): void => {
     vi.clearAllMocks();
-    vi.mocked(fetchDashboardSummary).mockResolvedValue({
-      metrics: {
-        activeAgents: 5,
-        tasksCompleted: 42,
-        totalTasks: 100,
-        tasksInProgress: 10,
-        activeProjects: 3,
-        errorRate: 0.5,
-      },
-      recentActivity: [],
-      activeJobs: [],
-      tokenBudget: [],
+  });
+
+  it("should render WidgetGrid with saved layout", async (): Promise<void> => {
+    vi.mocked(layoutsApi.fetchDefaultLayout).mockResolvedValue(mockExistingLayout);
+
+    render(<DashboardPage />);
+
+    await waitFor((): void => {
+      expect(screen.getByTestId("widget-grid")).toBeInTheDocument();
+    });
+
+    expect(screen.getByTestId("widget-TasksWidget-default")).toBeInTheDocument();
+    expect(screen.getByTestId("widget-CalendarWidget-default")).toBeInTheDocument();
+  });
+
+  it("should create default layout when none exists", async (): Promise<void> => {
+    vi.mocked(layoutsApi.fetchDefaultLayout).mockResolvedValue(null);
+    vi.mocked(layoutsApi.createLayout).mockResolvedValue({
+      ...mockExistingLayout,
+      layout: [{ i: "TasksWidget-default", x: 0, y: 0, w: 4, h: 2 }],
+    });
+
+    render(<DashboardPage />);
+
+    await waitFor((): void => {
+      expect(layoutsApi.createLayout).toHaveBeenCalledWith("ws-test-123", {
+        name: "Default",
+        isDefault: true,
+        layout: expect.arrayContaining([
+          expect.objectContaining({ i: "TasksWidget-default" }),
+        ]) as WidgetPlacement[],
+      });
     });
   });
 
-  it("should render the DashboardMetrics widget", async (): Promise<void> => {
+  it("should show loading spinner initially", (): void => {
+    vi.mocked(layoutsApi.fetchDefaultLayout).mockReturnValue(new Promise(() => {}));
+
     render(<DashboardPage />);
+
+    expect(screen.getByText("Loading dashboard...")).toBeInTheDocument();
+  });
+
+  it("should fall back to default layout on API error", async (): Promise<void> => {
+    vi.mocked(layoutsApi.fetchDefaultLayout).mockRejectedValue(new Error("Network error"));
+
+    render(<DashboardPage />);
+
     await waitFor((): void => {
-      expect(screen.getByTestId("dashboard-metrics")).toBeInTheDocument();
+      expect(screen.getByTestId("widget-grid")).toBeInTheDocument();
     });
   });
 
-  it("should render the OrchestratorSessions widget", async (): Promise<void> => {
+  it("should render Dashboard heading", async (): Promise<void> => {
+    vi.mocked(layoutsApi.fetchDefaultLayout).mockResolvedValue(mockExistingLayout);
+
     render(<DashboardPage />);
+
     await waitFor((): void => {
-      expect(screen.getByTestId("orchestrator-sessions")).toBeInTheDocument();
+      expect(screen.getByText("Dashboard")).toBeInTheDocument();
     });
   });
 
-  it("should render the QuickActions widget", async (): Promise<void> => {
+  it("should render Edit Layout button", async (): Promise<void> => {
+    vi.mocked(layoutsApi.fetchDefaultLayout).mockResolvedValue(mockExistingLayout);
+
     render(<DashboardPage />);
+
     await waitFor((): void => {
-      expect(screen.getByTestId("quick-actions")).toBeInTheDocument();
+      expect(screen.getByText("Edit Layout")).toBeInTheDocument();
     });
   });
 
-  it("should render the ActivityFeed widget", async (): Promise<void> => {
-    render(<DashboardPage />);
-    await waitFor((): void => {
-      expect(screen.getByTestId("activity-feed")).toBeInTheDocument();
-    });
-  });
+  it("should toggle edit mode on button click", async (): Promise<void> => {
+    vi.mocked(layoutsApi.fetchDefaultLayout).mockResolvedValue(mockExistingLayout);
 
-  it("should render the TokenBudget widget", async (): Promise<void> => {
     render(<DashboardPage />);
-    await waitFor((): void => {
-      expect(screen.getByTestId("token-budget")).toBeInTheDocument();
-    });
-  });
 
-  it("should render error state when API fails", async (): Promise<void> => {
-    vi.mocked(fetchDashboardSummary).mockRejectedValueOnce(new Error("Network error"));
-    render(<DashboardPage />);
     await waitFor((): void => {
-      expect(screen.getByText("Failed to load dashboard data")).toBeInTheDocument();
+      expect(screen.getByText("Edit Layout")).toBeInTheDocument();
     });
+
+    act((): void => {
+      screen.getByText("Edit Layout").click();
+    });
+
+    expect(screen.getByText("Done")).toBeInTheDocument();
+    expect(screen.getByTestId("widget-grid").getAttribute("data-editing")).toBe("true");
   });
 });
diff --git a/apps/web/src/app/(authenticated)/page.tsx b/apps/web/src/app/(authenticated)/page.tsx
index dbaf667..641627e 100644
--- a/apps/web/src/app/(authenticated)/page.tsx
+++ b/apps/web/src/app/(authenticated)/page.tsx
@@ -1,22 +1,24 @@
 "use client";
 
-import { useState, useEffect } from "react";
+import { useState, useEffect, useCallback, useRef } from "react";
 import type { ReactElement } from "react";
-import { DashboardMetrics } from "@/components/dashboard/DashboardMetrics";
-import { OrchestratorSessions } from "@/components/dashboard/OrchestratorSessions";
-import { QuickActions } from "@/components/dashboard/QuickActions";
-import { ActivityFeed } from "@/components/dashboard/ActivityFeed";
-import { TokenBudget } from "@/components/dashboard/TokenBudget";
-import { fetchDashboardSummary } from "@/lib/api/dashboard";
-import type { DashboardSummaryResponse } from "@/lib/api/dashboard";
+import type { WidgetPlacement } from "@mosaic/shared";
+import { WidgetGrid } from "@/components/widgets/WidgetGrid";
+import { DEFAULT_LAYOUT } from "@/components/widgets/defaultLayout";
+import { fetchDefaultLayout, createLayout, updateLayout } from "@/lib/api/layouts";
 import { useWorkspaceId } from "@/lib/hooks";
 
 export default function DashboardPage(): ReactElement {
   const workspaceId = useWorkspaceId();
-  const [data, setData] = useState<DashboardSummaryResponse | null>(null);
+  const [layout, setLayout] = useState<WidgetPlacement[]>(DEFAULT_LAYOUT);
+  const [layoutId, setLayoutId] = useState<string | null>(null);
+  const [isEditing, setIsEditing] = useState(false);
   const [isLoading, setIsLoading] = useState(true);
-  const [error, setError] = useState<string | null>(null);
 
+  // Debounce timer for auto-saving layout changes
+  const saveTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
+
+  // Load the user's default layout (or create one)
   useEffect(() => {
     if (!workspaceId) {
       setIsLoading(false);
@@ -24,70 +26,86 @@ export default function DashboardPage(): ReactElement {
     }
 
     const wsId = workspaceId;
-    let cancelled = false;
-    setError(null);
-    setIsLoading(true);
+    const ac = new AbortController();
 
-    async function loadSummary(): Promise<void> {
+    async function loadLayout(): Promise<void> {
       try {
-        const summary = await fetchDashboardSummary(wsId);
-        if (!cancelled) {
-          setData(summary);
+        const existing = await fetchDefaultLayout(wsId);
+        if (ac.signal.aborted) return;
+
+        if (existing) {
+          setLayout(existing.layout);
+          setLayoutId(existing.id);
+        } else {
+          const created = await createLayout(wsId, {
+            name: "Default",
+            isDefault: true,
+            layout: DEFAULT_LAYOUT,
+          });
+          // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition -- aborted can change during await
+          if (ac.signal.aborted) return;
+          setLayout(created.layout);
+          setLayoutId(created.id);
         }
       } catch (err: unknown) {
-        console.error("[Dashboard] Failed to fetch summary:", err);
-        if (!cancelled) {
-          setError("Failed to load dashboard data");
-        }
-      } finally {
-        if (!cancelled) {
-          setIsLoading(false);
-        }
+        console.error("[Dashboard] Failed to load layout:", err);
       }
+      setIsLoading(false);
     }
 
-    void loadSummary();
+    void loadLayout();
 
     return (): void => {
-      cancelled = true;
+      ac.abort();
     };
   }, [workspaceId]);
 
-  useEffect(() => {
-    if (!workspaceId) return;
+  // Save layout changes with debounce
+  const saveLayout = useCallback(
+    (newLayout: WidgetPlacement[]) => {
+      if (!workspaceId || !layoutId) return;
 
-    let cancelled = false;
-    const wsId = workspaceId;
+      if (saveTimerRef.current) {
+        clearTimeout(saveTimerRef.current);
+      }
 
-    const interval = setInterval(() => {
-      fetchDashboardSummary(wsId)
-        .then((summary) => {
-          if (!cancelled) setData(summary);
-        })
-        .catch((err: unknown) => {
-          console.error("[Dashboard] Refresh failed:", err);
+      saveTimerRef.current = setTimeout(() => {
+        void updateLayout(workspaceId, layoutId, { layout: newLayout }).catch((err: unknown) => {
+          console.error("[Dashboard] Failed to save layout:", err);
         });
-    }, 30_000);
+      }, 800);
+    },
+    [workspaceId, layoutId]
+  );
 
-    return (): void => {
-      cancelled = true;
-      clearInterval(interval);
-    };
-  }, [workspaceId]);
+  const handleLayoutChange = useCallback(
+    (newLayout: WidgetPlacement[]) => {
+      setLayout(newLayout);
+      saveLayout(newLayout);
+    },
+    [saveLayout]
+  );
+
+  const handleRemoveWidget = useCallback(
+    (widgetId: string) => {
+      const updated = layout.filter((item) => item.i !== widgetId);
+      setLayout(updated);
+      saveLayout(updated);
+    },
+    [layout, saveLayout]
+  );
 
   if (isLoading) {
     return (
-      <div style={{ display: "flex", flexDirection: "column", gap: 16 }}>
-        <DashboardMetrics />
-        <div className="dash-grid">
-          <div style={{ display: "flex", flexDirection: "column", gap: 16, minWidth: 0 }}>
-            <OrchestratorSessions />
-            <QuickActions />
-          </div>
-          <div style={{ display: "flex", flexDirection: "column", gap: 16 }}>
-            <ActivityFeed />
-            <TokenBudget />
-          </div>
+      <div className="flex items-center justify-center" style={{ minHeight: 400 }}>
+        <div className="flex flex-col items-center gap-2">
+          <div
+            className="w-8 h-8 border-2 border-t-transparent rounded-full animate-spin"
+            style={{ borderColor: "var(--primary)", borderTopColor: "transparent" }}
+          />
+          <span className="text-sm" style={{ color: "var(--muted)" }}>
+            Loading dashboard...
+          </span>
         </div>
       </div>
     );
@@ -95,32 +113,45 @@ export default function DashboardPage(): ReactElement {
 
   return (
     <div style={{ display: "flex", flexDirection: "column", gap: 16 }}>
-      {error && (
-        <div
+      {/* Dashboard header with edit toggle */}
+      <div className="flex items-center justify-between">
+        <h1
           style={{
-            padding: "12px 16px",
-            marginBottom: 16,
-            background: "rgba(229,72,77,0.1)",
-            border: "1px solid var(--border)",
-            borderRadius: "var(--r)",
+            fontSize: "1.5rem",
+            fontWeight: 700,
             color: "var(--text)",
-            fontSize: "0.85rem",
+            margin: 0,
           }}
         >
-          {error}
-        </div>
-      )}
-      <DashboardMetrics metrics={data?.metrics} />
-      <div className="dash-grid">
-        <div style={{ display: "flex", flexDirection: "column", gap: 16, minWidth: 0 }}>
-          <OrchestratorSessions jobs={data?.activeJobs} />
-          <QuickActions />
-        </div>
-        <div style={{ display: "flex", flexDirection: "column", gap: 16 }}>
-          <ActivityFeed items={data?.recentActivity} />
-          <TokenBudget budgets={data?.tokenBudget} />
-        </div>
+          Dashboard
+        </h1>
+        <button
+          onClick={() => {
+            setIsEditing((prev) => !prev);
+          }}
+          style={{
+            padding: "6px 14px",
+            borderRadius: "var(--r)",
+            border: isEditing ? "1px solid var(--primary)" : "1px solid var(--border)",
+            background: isEditing ? "var(--primary)" : "transparent",
+            color: isEditing ? "#fff" : "var(--text-2)",
+            fontSize: "0.83rem",
+            fontWeight: 500,
+            cursor: "pointer",
+            transition: "all 0.15s ease",
+          }}
+        >
+          {isEditing ? "Done" : "Edit Layout"}
+        </button>
       </div>
+
+      {/* Widget grid */}
+      <WidgetGrid
+        layout={layout}
+        onLayoutChange={handleLayoutChange}
+        {...(isEditing && { onRemoveWidget: handleRemoveWidget })}
+        isEditing={isEditing}
+      />
     </div>
   );
 }
diff --git a/apps/web/src/components/widgets/BaseWidget.tsx b/apps/web/src/components/widgets/BaseWidget.tsx
index aca0b57..04540c6 100644
--- a/apps/web/src/components/widgets/BaseWidget.tsx
+++ b/apps/web/src/components/widgets/BaseWidget.tsx
@@ -37,16 +37,31 @@ export function BaseWidget({
   return (
     <div
       data-widget-id={id}
-      className={cn(
-        "flex flex-col h-full bg-white rounded-lg border border-gray-200 shadow-sm overflow-hidden",
-        className
-      )}
+      className={cn("flex flex-col h-full overflow-hidden", className)}
+      style={{
+        background: "var(--surface)",
+        border: "1px solid var(--border)",
+        borderRadius: "var(--r-lg)",
+        boxShadow: "var(--shadow-sm)",
+      }}
     >
       {/* Widget Header */}
-      <div className="flex items-center justify-between px-4 py-3 border-b border-gray-100 bg-gray-50">
+      <div
+        className="flex items-center justify-between px-4 py-3"
+        style={{
+          borderBottom: "1px solid var(--border)",
+          background: "var(--surface-2)",
+        }}
+      >
         <div className="flex-1 min-w-0">
-          <h3 className="text-sm font-semibold text-gray-900 truncate">{title}</h3>
-          {description && <p className="text-xs text-gray-500 truncate mt-0.5">{description}</p>}
+          <h3 className="text-sm font-semibold truncate" style={{ color: "var(--text)" }}>
+            {title}
+          </h3>
+          {description && (
+            <p className="text-xs truncate mt-0.5" style={{ color: "var(--muted)" }}>
+              {description}
+            </p>
+          )}
         </div>
 
         {/* Control buttons - only show if handlers provided */}
@@ -56,7 +71,8 @@ export function BaseWidget({
               <button
                 onClick={onEdit}
                 aria-label="Edit widget"
-                className="p-1 text-gray-400 hover:text-gray-600 hover:bg-gray-100 rounded transition-colors"
+                className="p-1 rounded transition-colors"
+                style={{ color: "var(--muted)" }}
                 title="Edit widget"
               >
                 <Settings className="w-4 h-4" />
@@ -66,7 +82,8 @@ export function BaseWidget({
               <button
                 onClick={onRemove}
                 aria-label="Remove widget"
-                className="p-1 text-gray-400 hover:text-red-600 hover:bg-red-50 rounded transition-colors"
+                className="p-1 rounded transition-colors"
+                style={{ color: "var(--muted)" }}
                 title="Remove widget"
               >
                 <X className="w-4 h-4" />
@@ -81,15 +98,24 @@ export function BaseWidget({
         {isLoading ? (
           <div className="flex items-center justify-center h-full">
             <div className="flex flex-col items-center gap-2">
-              <div className="w-8 h-8 border-2 border-blue-500 border-t-transparent rounded-full animate-spin" />
-              <span className="text-sm text-gray-500">Loading...</span>
+              <div
+                className="w-8 h-8 border-2 border-t-transparent rounded-full animate-spin"
+                style={{ borderColor: "var(--primary)", borderTopColor: "transparent" }}
+              />
+              <span className="text-sm" style={{ color: "var(--muted)" }}>
+                Loading...
+              </span>
             </div>
           </div>
         ) : error ? (
           <div className="flex items-center justify-center h-full">
             <div className="text-center">
-              <div className="text-red-500 text-sm font-medium mb-1">Error</div>
-              <div className="text-xs text-gray-600">{error}</div>
+              <div className="text-sm font-medium mb-1" style={{ color: "var(--danger)" }}>
+                Error
+              </div>
+              <div className="text-xs" style={{ color: "var(--muted)" }}>
+                {error}
+              </div>
             </div>
           </div>
         ) : (
diff --git a/apps/web/src/components/widgets/WidgetGrid.tsx b/apps/web/src/components/widgets/WidgetGrid.tsx
index 4e597fe..6d6d7e0 100644
--- a/apps/web/src/components/widgets/WidgetGrid.tsx
+++ b/apps/web/src/components/widgets/WidgetGrid.tsx
@@ -5,7 +5,7 @@
 
 /* eslint-disable @typescript-eslint/no-non-null-assertion */
 
-import { useCallback, useMemo } from "react";
+import { useCallback, useMemo, useRef, useState, useEffect } from "react";
 import GridLayout from "react-grid-layout";
 import type { Layout, LayoutItem } from "react-grid-layout";
 import type { WidgetPlacement } from "@mosaic/shared";
@@ -33,6 +33,30 @@ export function WidgetGrid({
   isEditing = false,
   className,
 }: WidgetGridProps): React.JSX.Element {
+  // Measure container width for responsive grid
+  const containerRef = useRef<HTMLDivElement>(null);
+  const [containerWidth, setContainerWidth] = useState(1200);
+
+  useEffect(() => {
+    const el = containerRef.current;
+    if (!el) return;
+
+    const observer = new ResizeObserver((entries): void => {
+      const entry = entries[0];
+      if (entry) {
+        setContainerWidth(entry.contentRect.width);
+      }
+    });
+    observer.observe(el);
+
+    // Set initial width
+    setContainerWidth(el.clientWidth);
+
+    return (): void => {
+      observer.disconnect();
+    };
+  }, []);
+
   // Convert WidgetPlacement to react-grid-layout Layout format
   const gridLayout: Layout = useMemo(
     () =>
@@ -96,22 +120,34 @@ export function WidgetGrid({
   // Empty state
   if (layout.length === 0) {
     return (
-      <div className="flex items-center justify-center h-full min-h-[400px] bg-gray-50 rounded-lg border-2 border-dashed border-gray-300">
+      <div
+        ref={containerRef}
+        className="flex items-center justify-center h-full min-h-[400px]"
+        style={{
+          background: "var(--surface-2)",
+          borderRadius: "var(--r-lg)",
+          border: "2px dashed var(--border)",
+        }}
+      >
         <div className="text-center">
-          <p className="text-gray-500 text-lg font-medium">No widgets yet</p>
-          <p className="text-gray-400 text-sm mt-1">Add widgets to customize your dashboard</p>
+          <p className="text-lg font-medium" style={{ color: "var(--muted)" }}>
+            No widgets yet
+          </p>
+          <p className="text-sm mt-1" style={{ color: "var(--muted)", opacity: 0.7 }}>
+            Add widgets to customize your dashboard
+          </p>
         </div>
       </div>
     );
   }
 
   return (
-    <div className={cn("widget-grid-container", className)}>
+    <div ref={containerRef} className={cn("widget-grid-container", className)}>
       <GridLayout
         className="layout"
         layout={gridLayout}
         onLayoutChange={handleLayoutChange}
-        width={1200}
+        width={containerWidth}
         gridConfig={{
           cols: 12,
           rowHeight: 100,
diff --git a/apps/web/src/components/widgets/__tests__/WidgetGrid.test.tsx b/apps/web/src/components/widgets/__tests__/WidgetGrid.test.tsx
index 9e4f171..8d1fbe9 100644
--- a/apps/web/src/components/widgets/__tests__/WidgetGrid.test.tsx
+++ b/apps/web/src/components/widgets/__tests__/WidgetGrid.test.tsx
@@ -3,11 +3,20 @@
  * Following TDD - write tests first!
  */
 
-import { describe, it, expect, vi } from "vitest";
+import { describe, it, expect, vi, beforeAll } from "vitest";
 import { render, screen } from "@testing-library/react";
 import { WidgetGrid } from "../WidgetGrid";
 import type { WidgetPlacement } from "@mosaic/shared";
 
+// ResizeObserver is not available in jsdom
+beforeAll((): void => {
+  global.ResizeObserver = vi.fn().mockImplementation(() => ({
+    observe: vi.fn(),
+    unobserve: vi.fn(),
+    disconnect: vi.fn(),
+  }));
+});
+
 // Mock react-grid-layout
 vi.mock("react-grid-layout", () => ({
   default: ({ children }: { children: React.ReactNode }): React.JSX.Element => (
diff --git a/apps/web/src/components/widgets/defaultLayout.ts b/apps/web/src/components/widgets/defaultLayout.ts
new file mode 100644
index 0000000..5bad5be
--- /dev/null
+++ b/apps/web/src/components/widgets/defaultLayout.ts
@@ -0,0 +1,25 @@
+/**
+ * Default dashboard layout — used when a user has no saved layout.
+ *
+ * Widget ID format: "WidgetType-default" where the prefix before the
+ * first "-" must match a key in WidgetRegistry.
+ *
+ * Grid: 12 columns, 100px row height.
+ */
+
+import type { WidgetPlacement } from "@mosaic/shared";
+
+export const DEFAULT_LAYOUT: WidgetPlacement[] = [
+  // Row 0 — top row (3 widgets, 4 cols each)
+  { i: "TasksWidget-default", x: 0, y: 0, w: 4, h: 2, minW: 1, minH: 2, maxW: 4 },
+  { i: "CalendarWidget-default", x: 4, y: 0, w: 4, h: 2, minW: 2, minH: 2, maxW: 4 },
+  { i: "AgentStatusWidget-default", x: 8, y: 0, w: 4, h: 2, minW: 1, minH: 2, maxW: 3 },
+
+  // Row 2 — middle row
+  { i: "ActiveProjectsWidget-default", x: 0, y: 2, w: 4, h: 3, minW: 2, minH: 2, maxW: 4 },
+  { i: "TaskProgressWidget-default", x: 4, y: 2, w: 4, h: 2, minW: 1, minH: 2, maxW: 3 },
+  { i: "OrchestratorEventsWidget-default", x: 8, y: 2, w: 4, h: 2, minW: 1, minH: 2, maxW: 4 },
+
+  // Row 4 — bottom
+  { i: "QuickCaptureWidget-default", x: 4, y: 4, w: 4, h: 1, minW: 2, minH: 1, maxW: 4, maxH: 2 },
+];
diff --git a/apps/web/src/lib/api/layouts.ts b/apps/web/src/lib/api/layouts.ts
new file mode 100644
index 0000000..58cf469
--- /dev/null
+++ b/apps/web/src/lib/api/layouts.ts
@@ -0,0 +1,54 @@
+/**
+ * Layout API client — CRUD for user dashboard layouts
+ */
+
+import type { UserLayout, WidgetPlacement } from "@mosaic/shared";
+import { apiGet, apiPost, apiPatch } from "./client";
+
+export interface CreateLayoutPayload {
+  name: string;
+  isDefault?: boolean;
+  layout: WidgetPlacement[];
+  metadata?: Record<string, unknown>;
+}
+
+export interface UpdateLayoutPayload {
+  name?: string;
+  isDefault?: boolean;
+  layout?: WidgetPlacement[];
+  metadata?: Record<string, unknown>;
+}
+
+/**
+ * Fetch the user's default layout for the active workspace.
+ * Returns null if no layout exists (404).
+ */
+export async function fetchDefaultLayout(workspaceId: string): Promise<UserLayout | null> {
+  try {
+    return await apiGet<UserLayout>("/api/layouts/default", workspaceId);
+  } catch {
+    // 404 = no layout yet — not an error
+    return null;
+  }
+}
+
+/**
+ * Create a new layout.
+ */
+export async function createLayout(
+  workspaceId: string,
+  payload: CreateLayoutPayload
+): Promise<UserLayout> {
+  return apiPost<UserLayout>("/api/layouts", payload, workspaceId);
+}
+
+/**
+ * Update an existing layout (partial patch).
+ */
+export async function updateLayout(
+  workspaceId: string,
+  layoutId: string,
+  payload: UpdateLayoutPayload
+): Promise<UserLayout> {
+  return apiPatch<UserLayout>(`/api/layouts/${layoutId}`, payload, workspaceId);
+}
diff --git a/docs/TASKS.md b/docs/TASKS.md
index 1d73d77..f1b0790 100644
--- a/docs/TASKS.md
+++ b/docs/TASKS.md
@@ -8,8 +8,8 @@
 | TW-THM-001  | done        | Theme architecture — Create theme definition interface, theme registry, and 5 built-in themes (Dark, Light, Nord, Dracula, Solarized) as TS files          | #487  | web  | feat/ms18-theme-architecture     | TW-PLAN-001                                            | TW-THM-002,TW-THM-003                       | worker       | 2026-02-23 | 2026-02-23   | 30K      | ~15K | PR #493 merged                             |
 | TW-THM-002  | done        | ThemeProvider upgrade — Load themes dynamically from registry, apply CSS variables, support instant theme switching without page reload                    | #487  | web  | feat/ms18-theme-provider-upgrade | TW-THM-001                                             | TW-THM-003,TW-VER-002                       | worker       | 2026-02-23 | 2026-02-23   | 25K      | ~12K | PR #494 merged                             |
 | TW-THM-003  | done        | Theme selection UI — Settings page section with theme browser, live preview swatches, persist selection to UserPreference.theme via API                    | #487  | web  | feat/ms18-theme-selection-ui     | TW-THM-001,TW-THM-002                                  | TW-VER-002                                  | worker       | 2026-02-23 | 2026-02-23   | 25K      | ~10K | PR #495 merged                             |
-| TW-WDG-001  | not-started | Widget definition seeding — Seed 7 existing widgets into widget_definitions table with correct sizing constraints and configSchema                         | #488  | api  | TBD                              | TW-PLAN-001                                            | TW-WDG-002                                  | worker       | —          | —            | 15K      | —    |                                            |
-| TW-WDG-002  | not-started | Dashboard → WidgetGrid migration — Replace hardcoded dashboard layout with WidgetGrid, load/save layout via UserLayout API, default layout on first visit  | #488  | web  | TBD                              | TW-WDG-001                                             | TW-WDG-003,TW-WDG-004,TW-WDG-005            | worker       | —          | —            | 40K      | —    |                                            |
+| TW-WDG-001  | done        | Widget definition seeding — Seed 7 existing widgets into widget_definitions table with correct sizing constraints and configSchema                         | #488  | api  | feat/ms18-widget-seed            | TW-PLAN-001                                            | TW-WDG-002                                  | worker       | 2026-02-23 | 2026-02-23   | 15K      | ~8K  | PR #496 merged                             |
+| TW-WDG-002  | in-progress | Dashboard → WidgetGrid migration — Replace hardcoded dashboard layout with WidgetGrid, load/save layout via UserLayout API, default layout on first visit  | #488  | web  | feat/ms18-widget-grid-migration  | TW-WDG-001                                             | TW-WDG-003,TW-WDG-004,TW-WDG-005            | worker       | 2026-02-23 | —            | 40K      | —    |                                            |
 | TW-WDG-003  | not-started | Widget picker UI — Drawer/dialog to browse available widgets from registry, preview size/description, add to dashboard                                     | #488  | web  | TBD                              | TW-WDG-002                                             | TW-VER-001                                  | worker       | —          | —            | 25K      | —    |                                            |
 | TW-WDG-004  | not-started | Widget configuration UI — Per-widget settings dialog using configSchema, configure data source/filters/colors/title                                        | #488  | web  | TBD                              | TW-WDG-002                                             | TW-VER-001                                  | worker       | —          | —            | 30K      | —    |                                            |
 | TW-WDG-005  | not-started | Layout management UI — Save/rename/switch/delete layouts, reset to default. UI controls in dashboard header area                                           | #488  | web  | TBD                              | TW-WDG-002                                             | TW-VER-001                                  | worker       | —          | —            | 20K      | —    |                                            |
@@ -26,9 +26,9 @@
 | Metric        | Value                     |
 | ------------- | ------------------------- |
 | Total tasks   | 16                        |
-| Completed     | 4 (PLAN-001, THM-001–003) |
-| In Progress   | 0                         |
-| Remaining     | 12                        |
-| PRs merged    | #493, #494, #495          |
+| Completed     | 5 (PLAN-001, THM-001–003, WDG-001) |
+| In Progress   | 0                                   |
+| Remaining     | 11                                  |
+| PRs merged    | #493, #494, #495, #496              |
 | Issues closed | —                         |
 | Milestone     | MS18-ThemeWidgets         |