Refactor TUI exit keybinding: use Ctrl+D instead of double Ctrl+C

- Changed Ctrl+C to only copy selected text (does nothing if no selection)
- Added Ctrl+D for immediate exit without confirmation
- Removed double Ctrl+C to quit logic and quit-pending state
- Removed ESC to cancel quit logic
- Updated footer and success messages to reflect new keybindings
- Cleaned up unused CSS class for quit-pending state

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: tjazerzen

tui/components.py

@@ -19,7 +19,6 @@ class CustomFooter(Horizontal):
     def __init__(self, render_id: str = "", **kwargs):
         super().__init__(**kwargs)
         self.render_id = render_id
-        self._footer_text_widget: Optional[Static] = None
 
     def compose(self):
         self._footer_text_widget = Static(self.FOOTER_TEXT, classes="custom-footer-text")