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/lib/components/ui/switch/index.ts

@@ -0,0 +1,7 @@
+import Root from "./switch.svelte";
+
+export {
+	Root,
+	//
+	Root as Switch,
+};