fix: unify theme system - fix theme switching persistence

- Unified localStorage key to 'theme-preference' across all components
- Fixed header.tsx using wrong localStorage key ('theme' instead of 'theme-preference')
- Added localStorage hybrid persistence for instant theme changes
- Removed router.refresh() which was causing stale data revert
- Replaced Blue theme with Sepia
- Consolidated auth() calls to prevent race conditions
- Updated UserSettingsData types to include all themes

keep-notes/app/(main)/settings/ai/page.tsx

@@ -14,7 +14,7 @@ export default async function AISettingsPage() {
   const settings = await getAISettings()
 
   return (
-    <div className="container mx-auto py-8 max-w-4xl">
+    <div className="space-y-6">
       <AISettingsHeader />
       <AISettingsPanel initialSettings={settings} />
     </div>