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>

tests/e2e/dashboard.spec.ts

@@ -0,0 +1,26 @@
+import { expect, test } from "@playwright/test";
+
+test("dashboard happy path", async ({ page }) => {
+  await page.goto("/");
+
+  // Page title is present
+  await expect(page.getByRole("heading", { name: "Dashboard" })).toBeVisible();
+
+  // Quick-add FAB is present
+  await expect(page.getByRole("button", { name: "Quick add" })).toBeVisible();
+
+  // Widget card titles from registered manifests should appear (exact match inside main)
+  const main = page.getByRole("main");
+  await expect(main.getByText("Upcoming events", { exact: true })).toBeVisible();
+  await expect(main.getByText("List items", { exact: true })).toBeVisible();
+  await expect(main.getByText("Notes", { exact: true }).first()).toBeVisible();
+  await expect(main.getByText("Recent activity", { exact: true })).toBeVisible();
+
+  // No horizontal scroll on mobile viewport
+  await page.setViewportSize({ width: 375, height: 812 });
+  await page.goto("/");
+  await expect(page.getByRole("heading", { name: "Dashboard" })).toBeVisible();
+  const bodyWidth = await page.evaluate(() => document.body.scrollWidth);
+  const viewportWidth = await page.evaluate(() => window.innerWidth);
+  expect(bodyWidth).toBeLessThanOrEqual(viewportWidth + 2); // 2px tolerance for sub-pixel rendering
+});