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/_core/schema.ts

@@ -2,6 +2,7 @@ import type { AdapterAccountType } from "@auth/core/adapters";
 import {
   index,
   integer,
+  jsonb,
   pgEnum,
   pgTable,
   primaryKey,
@@ -23,6 +24,7 @@ export const users = pgTable("users", {
   image: text("image"),
   theme: text("theme").notNull().default("default"),
   themeMode: text("theme_mode").notNull().default("system"),
+  defaultDashboardLayout: jsonb("default_dashboard_layout"),
   createdAt: timestamp("created_at", { withTimezone: true }).notNull().defaultNow(),
 });