refactor: use configurable keybindings for all UI hints (#724)

Follow-up to #717. Replaces all remaining hardcoded keybinding hints with configurable ones.

- Add pasteImage to AppAction so it can be configured in keybindings.json
- Create keybinding-hints.ts with reusable helper functions:
  - editorKey(action) / appKey(keybindings, action) - get key display string
  - keyHint(action, desc) / appKeyHint(kb, action, desc) / rawKeyHint(key, desc) - styled hints
- Export helpers from components/index.ts for extensions
- Update all components to use configured keybindings
- Remove now-unused getDisplayString() from KeybindingsManager and EditorKeybindingsManager
- Use keybindings.matches() instead of matchesKey() for pasteImage in custom-editor.ts

packages/coding-agent/src/modes/interactive/components/index.ts

@@ -13,6 +13,7 @@ export { ExtensionEditorComponent } from "./extension-editor.js";
 export { ExtensionInputComponent } from "./extension-input.js";
 export { ExtensionSelectorComponent } from "./extension-selector.js";
 export { FooterComponent } from "./footer.js";
+export { appKey, appKeyHint, editorKey, keyHint, rawKeyHint } from "./keybinding-hints.js";
 export { LoginDialogComponent } from "./login-dialog.js";
 export { ModelSelectorComponent } from "./model-selector.js";
 export { OAuthSelectorComponent } from "./oauth-selector.js";