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>

package.json

@@ -12,11 +12,12 @@
   "scripts": {
     "start": "electron .",
     "start:dev": "FABRIC_DESKTOP_URL=http://localhost:5173 electron .",
-    "pack": "electron-builder --dir",
-    "dist": "electron-builder",
-    "dist:linux": "electron-builder --linux AppImage deb tar.gz",
-    "dist:mac": "electron-builder --mac dmg zip",
-    "dist:win": "electron-builder --win nsis zip"
+    "build:renderer": "node scripts/build-renderer.mjs",
+    "pack": "npm run build:renderer && electron-builder --dir",
+    "dist": "npm run build:renderer && electron-builder",
+    "dist:linux": "npm run build:renderer && electron-builder --linux AppImage deb tar.gz",
+    "dist:mac": "npm run build:renderer && electron-builder --mac dmg zip",
+    "dist:win": "npm run build:renderer && electron-builder --win nsis zip"
   },
   "devDependencies": {
     "electron": "^37.2.1",
@@ -34,7 +35,8 @@
       "preload.js",
       "offline.html",
       "package.json",
-      "assets/**"
+      "assets/**",
+      "renderer/**"
     ],
     "asar": true,
     "icon": "build/icon.png",