feat(ui): implement which-key popup for command completions

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)

src/ui/mod.rs

@@ -9,3 +9,4 @@ pub mod import_wizard_ui;
 pub mod panel;
 pub mod tile_bar;
 pub mod view_panel;
+pub mod which_key;