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/prisma/migrations/20260117200000_add_settings_fields/migration.sql

@@ -0,0 +1,4 @@
+-- AlterTable
+ALTER TABLE "UserAISettings" ADD COLUMN "emailNotifications" BOOLEAN NOT NULL DEFAULT false;
+ALTER TABLE "UserAISettings" ADD COLUMN "desktopNotifications" BOOLEAN NOT NULL DEFAULT false;
+ALTER TABLE "UserAISettings" ADD COLUMN "anonymousAnalytics" BOOLEAN NOT NULL DEFAULT false;