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-sync.ts

@@ -42,6 +42,7 @@ const SETTINGS_FIELDS_TO_SYNC = [
   'useWorktrees',
   'defaultPlanningMode',
   'defaultRequirePlanApproval',
+  'defaultFeatureModel',
   'muteDoneSound',
   'enhancementModel',
   'validationModel',
@@ -466,6 +467,7 @@ export async function refreshSettingsFromServer(): Promise<boolean> {
       useWorktrees: serverSettings.useWorktrees,
       defaultPlanningMode: serverSettings.defaultPlanningMode,
       defaultRequirePlanApproval: serverSettings.defaultRequirePlanApproval,
+      defaultFeatureModel: serverSettings.defaultFeatureModel ?? { model: 'opus' },
       muteDoneSound: serverSettings.muteDoneSound,
       enhancementModel: serverSettings.enhancementModel,
       validationModel: serverSettings.validationModel,