feat(viewer): group-by-project toggle + Gitea 404 handling

- static/board.js: full rerender on toggle-grouping; by-project mode = N cols
  per project with status-badge inside cards, sorted by canonical status order
- static/board.css: drop placeholder, add .badge.status
- src/gitea.ts: getLatestCommitIso returns null on 404 (path never existed
  remotely; seen on books repo where some per-task slugs in STATUS.md
  don't have file in default branch) + new test
- .gitignore: dist/ already present; add .playwright-mcp/, board-*.png

Smoke-verified: dist/index.html, 78 records from 4 repos, both toggle modes
work in browser, drawer lazy-fetches markdown.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/gitea.test.ts

@@ -100,6 +100,15 @@ describe('GiteaClient.getLatestCommitIso', () => {
 
     expect(await client.getLatestCommitIso('o', 'r', 'p.md')).toBeNull();
   });
+
+  test('returns null when Gitea responds 404 (path never existed)', async () => {
+    const mockFetch: MockFetch = vi.fn().mockResolvedValue(
+      new Response('Not Found', { status: 404 }),
+    );
+    const client = makeClient(mockFetch);
+
+    expect(await client.getLatestCommitIso('o', 'r', 'missing.md')).toBeNull();
+  });
 });
 
 describe('GiteaClient.rawUrl', () => {