feat(desktop): self-contained — bundle the frontend

- build:renderer builds Fabric.Frontend with relative base and copies
  the static bundle into renderer/; dist/pack scripts run it first.
- main.js: production loads renderer/index.html when present (falls
  back to offline.html); packaged app no longer needs a separate
  frontend server (still talks to Center/Guild backends, which the
  login screen points at — file:// origin is CORS-allowed).
- package files += renderer/**; gitignore renderer/.

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

.gitignore

@@ -1,2 +1,3 @@
 node_modules/
 dist/
+renderer/