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/App.vue

@@ -1,11 +1,7 @@
-<script setup lang="ts"></script>
+<script setup lang="ts">
+import { RouterView } from 'vue-router'
+</script>
 
 <template>
-  <h1>You did it!</h1>
-  <p>
-    Visit <a href="https://vuejs.org/" target="_blank" rel="noopener">vuejs.org</a> to read the
-    documentation
-  </p>
+  <RouterView />
 </template>
-
-<style scoped></style>