feat(mobile): bootstrap mobile app (PWA + Capacitor + backend auth/CORS)

Implements the §12 bootstrap from docs/mobile-app-plan.md.

Backend (§6):
- auth: optional returnToken flag on login returns the JWT in the body
  (data.authToken) for native Keychain/Keystore + Bearer; web cookie flow
  unchanged.
- main.ts: explicit CORS allowlist (APP_URL + CORS_ALLOWED_ORIGINS env +
  Capacitor WebView origins), credentials enabled, replaces open enableCors().
- optional OpenAPI/Swagger at /api/docs behind SWAGGER_ENABLED.
- env: CORS_ALLOWED_ORIGINS, SWAGGER_ENABLED, CAP_SERVER_URL.

PWA:
- manifest metadata, hand-rolled service worker (network-first nav, SWR
  assets, never intercepts /api,/socket.io,/collab), prod-only registration,
  apple-touch-icon.

Capacitor:
- capacitor.config.ts (webDir apps/client/dist; iOS via CAP_SERVER_URL to
  avoid bundling the AGPL client in the .ipa, see plan §9), cap:* scripts,
  deps, .gitignore for native dirs.
- docs/mobile-bootstrap.md documenting what is done and the remaining manual
  steps (cap add ios/android, APNs/FCM, stores).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/package.json

@@ -64,6 +64,7 @@
     "@nestjs/platform-fastify": "^11.1.19",
     "@nestjs/platform-socket.io": "^11.1.19",
     "@nestjs/schedule": "^6.1.3",
+    "@nestjs/swagger": "^11.2.0",
     "@nestjs/terminus": "^11.1.1",
     "@nestjs/throttler": "^6.5.0",
     "@nestjs/websockets": "^11.1.19",