Change Ctrl+C behavior to match other terminal AI tools (claude, codex, pi):
- First Ctrl+C clears the current input when text is present
- Second Ctrl+C (within 3 seconds) quits the application
- Ctrl+C on empty input quits immediately
- 3-second auto-reset timer clears the 'pressed once' state
- Flag also resets after message submission
Updates placeholder text and help message to reflect new behavior.
Fixes#13