fix: stabilize macOS clipboard decoding and status updates

[callzhang]

Summary

• harden the macOS sync envelope decoder so Android clipboard updates without AES metadata no longer crash while removing the redundant padding/print logic
• mark incoming WebSocket connections with the decoded device id and broadcast online status changes so paired devices report their reachability again

Testing

• swift test (fails: AppKit is unavailable on this Linux runner)

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Codex Task

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Note

Refactors Base64 decoding in SyncEngine to be safe and reusable, and updates connection metadata + online status when clipboard frames arrive over LAN WebSocket.

• SyncEngine:
  • Introduces addBase64Padding and decodeBase64Field to safely decode Android Base64 (handles missing padding, empty strings).
  • Refactors SyncEnvelope.Payload and EncryptionMetadata custom decoders to use the helper; removes verbose debug/padding logic.
• TransportManager:
  • On didReceiveClipboardData, decodes envelope header to attach deviceId to the connection and posts DeviceConnectionStatusChanged (isOnline: true).

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