feat: add Force Quit command with Ctrl+Q shortcut

[linw1995]

Summary

Add a Force Quit command to the TUI that can be triggered via Ctrl+Q, providing an alternative exit method to Ctrl+C.

Changes

• actions.rs: Add ForceQuit variant to CommandPaletteAction
• app.rs: Handle ForceQuit action by setting should_quit flag
• chat.rs:
  • Introduce ExitShortcut enum to track different exit shortcuts (Ctrl+C/Ctrl+Q)
  • Refactor handle_exit_shortcut to support multiple shortcuts
  • Update cancellation guard to track last used shortcut
  • Improve user prompts to show correct shortcut names
• command_palette.rs:
  • Add Force Quit command with Ctrl-q shortcut
  • Wire up ForceQuit action in command handler

Behavior

• Ctrl+C: Existing behavior - double press to cancel/exit
• Ctrl+Q: New Force Quit command - also double press to cancel/exit
• Both shortcuts show appropriate prompts based on which key was pressed

This provides users with a dedicated force quit option independent of the terminal's SIGINT handling.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.60%. Comparing base (a46f02c) to head (a5cfed3).
⚠️ Report is 15 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/coco-tui/src/components/chat.rs	0.00%	30 Missing ⚠️
crates/coco-tui/src/components/command_palette.rs	0.00%	6 Missing ⚠️
crates/coco-tui/src/app.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #159      +/-   ##
==========================================
- Coverage   50.22%   47.60%   -2.63%     
==========================================
  Files          93      100       +7     
  Lines       25033    27308    +2275     
==========================================
+ Hits        12574    12999     +425     
- Misses      12459    14309    +1850     

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