feat: enhance settings view and feature defaults management

- Introduced default feature model settings in the settings view, allowing users to specify the default AI model for new feature cards.
- Updated navigation to include a direct link to model defaults in the settings menu.
- Enhanced the Add Feature dialog to utilize the default feature model from the app store.
- Implemented synchronization of the default feature model in settings migration and sync hooks.
- Improved UI components to reflect changes in default settings, ensuring a cohesive user experience.

libs/types/src/settings.ts

@@ -375,6 +375,8 @@ export interface GlobalSettings {
   defaultPlanningMode: PlanningMode;
   /** Default: require manual approval before generating */
   defaultRequirePlanApproval: boolean;
+  /** Default model and thinking level for new feature cards */
+  defaultFeatureModel: PhaseModelEntry;
 
   // Audio Preferences
   /** Mute completion notification sound */
@@ -698,6 +700,7 @@ export const DEFAULT_GLOBAL_SETTINGS: GlobalSettings = {
   useWorktrees: true,
   defaultPlanningMode: 'skip',
   defaultRequirePlanApproval: false,
+  defaultFeatureModel: { model: 'opus' },
   muteDoneSound: false,
   phaseModels: DEFAULT_PHASE_MODELS,
   enhancementModel: 'sonnet',