feat: resilient camera reconnection with exponential backoff

[johnoct]

Summary

• Tolerate 30 dropped frames (~3s at 10 FPS) before treating camera as lost, instead of the old 3-frame instant teardown
• Add try_open_camera() that returns None instead of sys.exit(1) on failure
• Add reconnect_camera() with exponential backoff (2s, 4s, 8s... capped at 60s, up to 10 attempts)
• Send macOS notifications on camera disconnect/reconnect so the user knows what's happening
• Graceful exit if all reconnect attempts exhausted

Test plan

• TestTryOpenCamera: returns cap when available, returns None when unavailable, tries AVFoundation fallback
• TestReconnectCamera: succeeds on first try, retries with backoff, returns None after max attempts, backoff timing verified, 60s cap verified
• All 60 tests pass (52 babyping + 8 web), zero regressions
• Manual: test with iPhone Continuity Camera at distance to verify reconnection works