fix: stabilize mac clipboard sync metadata

[callzhang]

Summary

• allow the macOS sync engine to treat empty nonce/tag values from Android as valid plain text frames so clipboard updates decode again
• keep LAN WebSocket connection metadata in sync with the device IDs carried by clipboard envelopes and expose a delegate callback when the device is identified
• post device connection status updates when a device is identified and ensure connection teardown still reports the device before metadata is discarded

Testing

• cd macos && swift test (fails: missing macOS AppKit in container)

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Codex Task

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Note

Treat empty nonce/tag as plaintext in sync, propagate device identity via WebSocket envelopes, and introduce a prober to infer/publish device online status from LAN activity.

• macOS Services:
  • ConnectionStatusProber (new):
    • Periodically infers device online/offline from last activity and LAN discovery, with manual overrides, and posts DeviceConnectionStatusChanged.
    • Tracks LAN peer add/remove, activity pings, and provides immediate status publishing; started/stopped by TransportManager.
  • TransportManager:
    • Integrates ConnectionStatusProber; records activity on clipboard/device events and LAN discovery changes; replaces direct status notifications.
    • Registers notification observer for ClipboardReceivedFromDevice; cleans up observers on deinit.
  • LanWebSocketServer:
    • Keeps connectionMetadata in sync with clipboard envelope deviceId and adds delegate callback didIdentifyDevice when device is determined.
    • updateConnectionMetadata now returns metadata; adjusts close ordering to notify delegate before clearing metadata.
  • SyncEngine:
    • Decodes empty encryption.nonce/tag as plaintext mode (no decryption), restoring clipboard decode compatibility with Android.

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