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/modules/index.ts

@@ -1,8 +1,10 @@
 import { registerModule } from "./_core/registry";
+import coreManifest from "./_core/manifest";
 import calendarManifest from "./calendar/manifest";
 import listsManifest from "./lists/manifest";
 import notesManifest from "./notes/manifest";
 
+registerModule(coreManifest);
 registerModule(calendarManifest);
 registerModule(listsManifest);
 registerModule(notesManifest);