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.

frontend-nuxt/composables/useFormSubmit.ts

@@ -0,0 +1,24 @@
+/**
+ * Composable for form submit with loading and error state.
+ * Use run(fn) to execute an async action; loading and error are updated automatically.
+ */
+export function useFormSubmit(defaultError = 'Something went wrong') {
+  const loading = ref(false)
+  const error = ref('')
+
+  async function run<T>(fn: () => Promise<T>): Promise<T | undefined> {
+    error.value = ''
+    loading.value = true
+    try {
+      const result = await fn()
+      return result
+    } catch (e: unknown) {
+      error.value = e instanceof Error ? e.message : defaultError
+      return undefined
+    } finally {
+      loading.value = false
+    }
+  }
+
+  return { loading, error, run }
+}