feat(web): migrate dashboard to WidgetGrid with layout persistence (#497)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.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);
+}