Implement `WhichKey` popup to show available command completions.
- Added `format_key_label` to convert `KeyCode` to human-readable strings.
- Added `binding_hints` to `Keymap` to extract available commands for a
given prefix.
- Added `src/ui/which_key.rs` for the widget implementation.
- Updated `src/ui/mod.rs` to export the new module.
Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)
