Skip content transfer and verification for empty files, sending
immediate confirmation from receiver.

- Update client and server registry logic
- Improve TCP connection instance handling

- Update challenge method to return (bool, String) tuple - Include key
ID in both success and failure cases - Update tests to verify key ID
matches expected value - Maintain same verification logic but provide
additional context

- Add Authentication, Locked, NoResult, and NotFound error variants -
Remove Crypto error variant - Reorder all variants in A-Z order for
better maintainability

- Fix let_underscore_future warning by properly awaiting async functions
- Make accept_import function async to match add_mapping usage
- Propagate errors properly with ? operator instead of ignoring them
- Replace manual Default implementation with derive attribute
- Replace vec! with array literal to avoid useless_vec warning
- All tests pass and code is now Clippy clean

- Enhance action_gen macro functionality
- Add new error variants for TCP connection