add auto-signup login and localized auth UI

- backend: userLogin auto-registers missing users and returns auto_registered
- frontend: add responsive auth page (login + signup flow via userLogin) with light/dark mode and logo
- i18n: wire language switching, add translations for cs/en/es/de
- ui/tooling: add Font Awesome integration

frontend/src/main.ts

@@ -1,9 +1,14 @@
 import { createApp } from 'vue'
+import { FontAwesomeIcon } from '@fortawesome/vue-fontawesome'
+import './assets/css/style.css'
 import App from './App.vue'
+import i18n from './i18n'
 import router from './router'
 
 const app = createApp(App)
 
+app.component('font-awesome-icon', FontAwesomeIcon)
 app.use(router)
+app.use(i18n)
 
 app.mount('#app')