Harden mobile screen sharing lifecycle

[phucnguyen1707]

Summary

• add a lifecycle-managed Android screen capture hook with guarded start, publish, stop, rollback, and unmount handling
• make the host WebRTC layer own screen-share senders per viewer without replacing microphone or relayed-audio tracks
• prevent stale async capture and signaling callbacks from restoring stopped sessions
• keep the host in-session when the server rejects an end request and show user-friendly publish failures
• document Android support and the current iOS ReplayKit limitation

Verification

• pnpm check:mobile
• 13 test files, 86 tests passed
• mobile lint and TypeScript checks passed
• Android Metro production bundle completed
• React Native dependency check confirmed one React runtime

Device coverage

The Android native path is covered by unit/integration tests and a production Metro bundle. A physical Android device run is still recommended during review because display-capture behavior depends on the OS media-projection prompt. iOS publishing remains explicitly unavailable until ReplayKit support is implemented.