Fix redirect loops and SSR hydration issues in auth flow

- Replace ref with useState in useAuth for SSR compatibility
- Move navigation logic from top-level to onMounted hooks
- Add guest middleware to login page to prevent auth conflicts
- Simplify dashboard auth checks by relying on middleware
- Add loading state to index page during auth resolution

This prevents infinite redirect loops and hydration mismatches that
occurred during server-side rendering when navigating between
authenticated and unauthenticated states.

composables/useAuth.ts

@@ -1,7 +1,8 @@
 import type { User } from '~/utils/types';
 
 export const useAuth = () => {
-  const user = ref<User | null>(null);
+  // Use useState for SSR compatibility - prevents hydration mismatches
+  const user = useState<User | null>('auth.user', () => null);
   const isAuthenticated = computed(() => !!user.value);
   const loading = ref(false);
   const error = ref<string | null>(null);