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.

apps/ui/src/hooks/use-settings-migration.ts

@@ -562,6 +562,7 @@ export function hydrateStoreFromSettings(settings: GlobalSettings): void {
     useWorktrees: settings.useWorktrees ?? true,
     defaultPlanningMode: settings.defaultPlanningMode ?? 'skip',
     defaultRequirePlanApproval: settings.defaultRequirePlanApproval ?? false,
+    defaultFeatureModel: settings.defaultFeatureModel ?? { model: 'opus' },
     muteDoneSound: settings.muteDoneSound ?? false,
     enhancementModel: settings.enhancementModel ?? 'sonnet',
     validationModel: settings.validationModel ?? 'opus',