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/package.json

@@ -15,7 +15,11 @@
     "format": "prettier --write --experimental-cli src/"
   },
   "dependencies": {
+    "@fortawesome/fontawesome-svg-core": "^7.2.0",
+    "@fortawesome/free-solid-svg-icons": "^7.2.0",
+    "@fortawesome/vue-fontawesome": "^3.1.3",
     "vue": "^3.5.27",
+    "vue-i18n": "^11.2.8",
     "vue-router": "^5.0.1"
   },
   "devDependencies": {