Implement dashboard composition (task 20)

- Add default_dashboard_layout jsonb to users + migration 0007
- Create _core/manifest.tsx with core.activity placeholder widget
- Register core manifest alongside calendar/lists/notes
- Update all three module manifests with real async server-component
  widget renders (upcoming events, month view, list items, notes)
- Add src/lib/dashboard.ts: computeDefaultLayout greedy packer +
  parseDashboardLayout Zod validator
- Build src/app/page.tsx: 12-col CSS Grid, static smColSpan lookup,
  per-widget Suspense for parallel loading, generic widget.render()
  dispatch — no widgetId branches
- Add tests/e2e/dashboard.spec.ts; all 4 E2E specs pass

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

src/lib/dashboard.ts

@@ -0,0 +1,67 @@
+import { z } from "zod";
+import { getRegistry } from "@/modules/_core";
+
+export type WidgetPlacement = {
+  widgetId: string;
+  config: unknown;
+  x: number;
+  y: number;
+  w: number;
+  h: number;
+};
+
+export type DashboardLayout = {
+  version: 1;
+  widgets: WidgetPlacement[];
+};
+
+const placementSchema = z.object({
+  widgetId: z.string(),
+  config: z.unknown(),
+  x: z.number().int().min(0),
+  y: z.number().int().min(0),
+  w: z.number().int().min(1).max(12),
+  h: z.number().int().min(1),
+});
+
+const layoutSchema = z.object({
+  version: z.literal(1),
+  widgets: z.array(placementSchema),
+});
+
+export function parseDashboardLayout(raw: unknown): DashboardLayout | null {
+  const result = layoutSchema.safeParse(raw);
+  if (!result.success) return null;
+  return result.data;
+}
+
+export function computeDefaultLayout(): DashboardLayout {
+  const { widgets } = getRegistry();
+  const sorted = [...widgets].sort((a, b) => a.defaultPriority - b.defaultPriority);
+
+  const placements: WidgetPlacement[] = [];
+  let curX = 0;
+  let curY = 0;
+  let rowH = 0;
+
+  for (const widget of sorted) {
+    const { w, h } = widget.defaultSize;
+    if (curX + w > 12) {
+      curY += rowH;
+      curX = 0;
+      rowH = 0;
+    }
+    placements.push({
+      widgetId: widget.id,
+      config: widget.defaultConfig,
+      x: curX,
+      y: curY,
+      w,
+      h,
+    });
+    curX += w;
+    rowH = Math.max(rowH, h);
+  }
+
+  return { version: 1, widgets: placements };
+}