Stabilize mobile realtime lifecycle handling

[phucnguyen1707]

Summary

• suspend host, viewer, chat, screen-share, SSE, WebRTC, media, and stats resources safely when the mobile app backgrounds
• reconnect exactly once on foreground and replace stalled SSE sessions with a heartbeat watchdog
• guard asynchronous signaling and media callbacks by connection generation so retired peers cannot restore stale state
• preserve microphone mute intent across reconnects and make screen-share publication and teardown serialized
• document the foreground call-continuity behavior and native screen-share ownership

CI hygiene

• apply the pinned Prettier formatter to two pre-existing unformatted web files that caused the repository-wide format check to fail
• keep the formatting-only commit separate from the mobile lifecycle commit; it has no runtime or type-semantic changes

Validation

• Node.js 24.9.0
• pnpm --filter @pairux/mobile lint
• pnpm --filter @pairux/mobile typecheck
• pnpm --filter @pairux/mobile test (13 files, 110 tests)
• pnpm --filter @pairux/shared-types build
• pnpm --filter @pairux/mobile verify:bundle
• Android production export completed and contained exactly one React 18.3.1 runtime
• pnpm format:check
• pnpm lint
• pnpm typecheck
• pnpm test
• pnpm build