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/layout.tsx

@@ -0,0 +1,25 @@
+'use client'
+
+import { SettingsNav } from '@/components/settings'
+
+export default function SettingsLayout({
+    children,
+}: {
+    children: React.ReactNode
+}) {
+    return (
+        <div className="container mx-auto py-10 px-4 max-w-6xl">
+            <div className="grid grid-cols-1 lg:grid-cols-4 gap-6">
+                {/* Sidebar Navigation */}
+                <aside className="lg:col-span-1">
+                    <SettingsNav />
+                </aside>
+
+                {/* Main Content */}
+                <main className="lg:col-span-3 space-y-6">
+                    {children}
+                </main>
+            </div>
+        </div>
+    )
+}