Document Android OAuth bridge page flow ...

- docs/auth.md: describe browser vs Android WebView flows, bridge page
  mechanism, state metadata detection via NaviAndroid UA
- docs/api.md: add /auth/mobile-done endpoint reference
- docs/android-client.md: add OAuth/login section covering intent filter,
  CookieManager injection, deep-link handling in onCreate/onNewIntent

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Open artifact links externally on Android

Add Android client documentation ...

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>