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>

static/board.css

@@ -242,16 +242,6 @@ aside.drawer[hidden] {
   padding: 0;
 }
 
-body.group-by-project main.board {
-  grid-template-columns: 1fr;
-}
-
-body.group-by-project main.board .col {
-  display: none;
-}
-
-body.group-by-project main.board::after {
-  content: "grouping by project: not implemented yet";
-  color: var(--muted);
-  padding: 1rem;
+.badge.status {
+  background: var(--badge-bg);
 }