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.

.cursor/rules/frontend-nuxt.mdc

@@ -7,5 +7,5 @@ alwaysApply: false
 # Frontend (Nuxt 3)
 
 - All frontend source lives in **frontend-nuxt/** (pages, components, composables, layouts, plugins, `public/gfx`). Production build output goes to `frontend/` and is served by the Go backend.
-- Public API to backend: use composables — e.g. `composables/useMapApi.ts`, `useAuth.ts`, `useAdminApi.ts` — not raw fetch in components.
+- Public API to backend: use `useMapApi()` from `composables/useMapApi.ts` for all API calls (auth, admin, config, maps, etc.); do not use raw fetch in components.
 - Nuxt 3 conventions; ensure dev proxy in `nuxt.config.ts` points at backend when running locally (e.g. 3080 or 8080).