fix: file transfer system critical bugs and add persistence

- Fix recovery logic: save transfer data before cleanup so recovery
  can actually use it to restart transfers
- Register GMCPClientFile package so Client.File.Download messages
  are handled (was silently dropped)
- Fix error handling in handleIncomingChunk: capture hash from header
  so errors report actual hash instead of "unknown"
- Add error handling to acceptTransfer in client.ts
- Wire up FileTransferStore for IndexedDB persistence:
  - Chunks saved as they arrive for resumable transfers
  - Metadata tracked for incomplete transfers
  - Cleanup on completion, hash mismatch, or cancellation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]> 11fcd43

Author: ctoth

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