feat(web): migrate dashboard to WidgetGrid with layout persistence

Replace the hardcoded dashboard layout with the WidgetGrid system.
The dashboard now loads/saves user layouts via the UserLayout API
and creates a default layout on first visit with all 7 widgets.

Changes:
- Add layout API client (fetchDefaultLayout, createLayout, updateLayout)
- Add default layout constant with 7-widget arrangement (12-col grid)
- Update BaseWidget + WidgetGrid to use CSS variables for theme compat
- Add responsive container width measurement via ResizeObserver
- Rewrite dashboard page to use WidgetGrid with edit mode toggle
- Update all tests for the new dashboard architecture

Ref: #488

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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);
+}