Implement user settings and dark mode toggle in layout component

- Added functionality to toggle dark mode based on user preferences, with state management and API integration for saving settings.
- Introduced a user avatar display with initials fallback and a dropdown menu for logout and dark mode options.
- Enhanced layout structure to conditionally render the header based on user authentication status.

src/routes/+layout.server.ts

@@ -0,0 +1,7 @@
+import type { LayoutServerLoad } from './$types';
+
+export const load: LayoutServerLoad = async ({ locals }) => {
+	return {
+		user: locals.user ? structuredClone(locals.user) : null
+	};
+};