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/debug-theme.tsx

@@ -0,0 +1,9 @@
+'use client'
+
+export function DebugTheme({ theme }: { theme: string }) {
+    return (
+        <div className="fixed bottom-4 left-4 z-50 bg-black text-white p-2 rounded text-xs opacity-80 pointer-events-none">
+            Debug Theme: {theme}
+        </div>
+    )
+}