Add i18n, routing and dark-mode support

Introduce internationalization and basic routing: add i18n initialization (src/i18n.js), translation files (public/i18n/en.json, de.json), and wire i18n into the app (import in main.jsx). Update App.jsx to use react-router (BrowserRouter, Routes) and react-i18next (useTranslation), provide simple Landing/Login/Imprint pages, a language toggle and a dark-mode toggle. Add dependencies in package.json (i18next, i18next-http-backend, react-i18next, react-router-dom) and small Tailwind style tweak (custom-variant for dark) in src/style.css.

package.json

@@ -11,8 +11,12 @@
   "dependencies": {
     "@mdi/font": "~7.4.47",
     "@tailwindcss/vite": "~4.2.2",
+    "i18next": "~26.0.0",
+    "i18next-http-backend": "~3.0.2",
     "react": "^19.2.4",
-    "react-dom": "^19.2.4"
+    "react-dom": "^19.2.4",
+    "react-i18next": "~17.0.0",
+    "react-router-dom": "~7.13.2"
   },
   "devDependencies": {
     "@vitejs/plugin-react": "^6.0.1",