feat(ui): add graceful shutdown and cache export

[olalie]

Summary

• Add graceful shutdown with Exit (X) and shutdown confirmation dialog
• Replace "Show cache in terminal" with downloadable cache export
• Improve application logging and remove temporary diagnostics
• Update help, README and related tests

Testing

• 308/308 tests passing
• Manually tested on Windows
  • Exit (X)
  • Cache export
  • Logging