Refactor frontend components and enhance API integration

- Updated frontend-nuxt.mdc to specify usage of composables for API calls.
- Added new AuthCard and ConfirmModal components for improved UI consistency.
- Introduced UserAvatar component for user profile display, replacing previous Gravatar implementation.
- Implemented useFormSubmit composable for handling form submissions with loading and error states.
- Enhanced vitest.config.ts to include coverage reporting for composables and components.
- Removed deprecated useAdminApi and useAuth composables to streamline API interactions.
- Updated login and setup pages to utilize new components and composables for better user experience.

internal/app/topic.go

@@ -32,6 +32,8 @@ func (t *Topic[T]) Send(b *T) {
 
 // Close closes all subscriber channels.
 func (t *Topic[T]) Close() {
+	t.mu.Lock()
+	defer t.mu.Unlock()
 	for _, c := range t.c {
 		close(c)
 	}