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)/admin/layout.tsx

@@ -0,0 +1,23 @@
+import { AdminSidebar } from '@/components/admin-sidebar'
+import { AdminContentArea } from '@/components/admin-content-area'
+import { auth } from '@/auth'
+import { redirect } from 'next/navigation'
+
+export default async function AdminLayout({
+  children,
+}: {
+  children: React.ReactNode
+}) {
+  const session = await auth()
+
+  if ((session?.user as any)?.role !== 'ADMIN') {
+    redirect('/')
+  }
+
+  return (
+    <div className="flex min-h-screen bg-gray-50 dark:bg-zinc-950">
+      <AdminSidebar />
+      <AdminContentArea>{children}</AdminContentArea>
+    </div>
+  )
+}