Add keybinds-rs crate with crossterm and serde features to provide
robust multi-key sequence support and pattern-based key matching.
Also update strum to 0.27 for improved enum derivation capabilities.

Replace custom key parsing logic with keybinds-rs, providing:
- Multi-key sequence support (e.g., 'g g', 'Ctrl+x Ctrl+c')
- Simplified dispatch API via KeybindingMode
- Consistent key string format across all platforms
- Removed parse.rs module in favor of keybinds-rs parser

The new system maintains all existing keybindings while enabling
advanced features like multi-key sequences and custom patterns.

Update TUI integration to use the new keybindings dispatch API:
- Call dispatch() instead of get_action() with KeyCode/KeyModifiers
- Use is_sequence_ongoing() to handle multi-key sequences
- Pass crossterm KeyEvent directly to dispatch method
- Minor help text formatting updates

These changes maintain the existing functionality while supporting
the new multi-key sequence capabilities.

Add detailed documentation for the new custom keybindings feature:
- Keybinding syntax guide (single keys, modifiers, sequences, special keys)
- Complete list of available modes with descriptions
- Example configuration showing common patterns
- Notes on key format conventions

Also update navigation keybinding descriptions in the shortcuts table
for clarity and consistency.