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/command/keymap.rs

@@ -8,6 +8,29 @@ use crate::ui::app::AppMode;
 use crate::ui::effect::Effect;
 
 use super::cmd::{self, CmdContext, CmdRegistry};
+
+/// Format a KeyCode as a short human-readable label for which-key display.
+fn format_key_label(code: &KeyCode) -> String {
+    match code {
+        KeyCode::Char(c) => c.to_string(),
+        KeyCode::Enter => "Enter".to_string(),
+        KeyCode::Esc => "Esc".to_string(),
+        KeyCode::Tab => "Tab".to_string(),
+        KeyCode::BackTab => "S-Tab".to_string(),
+        KeyCode::Backspace => "BS".to_string(),
+        KeyCode::Delete => "Del".to_string(),
+        KeyCode::Left => "←".to_string(),
+        KeyCode::Right => "→".to_string(),
+        KeyCode::Up => "↑".to_string(),
+        KeyCode::Down => "↓".to_string(),
+        KeyCode::Home => "Home".to_string(),
+        KeyCode::End => "End".to_string(),
+        KeyCode::PageUp => "PgUp".to_string(),
+        KeyCode::PageDown => "PgDn".to_string(),
+        KeyCode::F(n) => format!("F{n}"),
+        _ => format!("{code:?}"),
+    }
+}
 // `cmd` module imported for `default_registry()` in default_keymaps()
 
 /// A key pattern that can be matched against a KeyEvent.
@@ -146,6 +169,35 @@ impl Keymap {
             .insert(KeyPattern::Any, Binding::Cmd { name, args: vec![] });
     }
 
+    /// Return human-readable hints for all concrete bindings in this keymap.
+    /// Used by the which-key popup to show available completions after a prefix key.
+    pub fn binding_hints(&self) -> Vec<(String, &'static str)> {
+        let mut hints: Vec<(String, &'static str)> = self
+            .bindings
+            .iter()
+            .filter_map(|(pattern, binding)| {
+                let label = match pattern {
+                    KeyPattern::Key(code, _) => format_key_label(code),
+                    KeyPattern::AnyChar | KeyPattern::Any => return None,
+                };
+                let name = match binding {
+                    Binding::Cmd { name, .. } => *name,
+                    Binding::Prefix(_) => return None,
+                    Binding::Sequence(steps) => {
+                        if let Some((name, _)) = steps.first() {
+                            *name
+                        } else {
+                            return None;
+                        }
+                    }
+                };
+                Some((label, name))
+            })
+            .collect();
+        hints.sort_by(|a, b| a.0.cmp(&b.0));
+        hints
+    }
+
     /// Look up the binding for a key.
     /// For Char keys, if exact (key, mods) match fails, retries with NONE
     /// modifiers since terminals vary in whether they send SHIFT for