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/components/admin-content-area.tsx

@@ -0,0 +1,19 @@
+import { cn } from '@/lib/utils'
+
+export interface AdminContentAreaProps {
+  children: React.ReactNode
+  className?: string
+}
+
+export function AdminContentArea({ children, className }: AdminContentAreaProps) {
+  return (
+    <main
+      className={cn(
+        'flex-1 bg-gray-50 dark:bg-zinc-950 p-6 overflow-auto',
+        className
+      )}
+    >
+      {children}
+    </main>
+  )
+}