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.

src/i18n.js

@@ -0,0 +1,19 @@
+import i18n from 'i18next';
+import {initReactI18next} from 'react-i18next';
+import HttpBackend from 'i18next-http-backend';
+
+
+i18n.use(HttpBackend)
+    .use(initReactI18next)
+    .init({
+        lng: 'de',
+        fallbackLng: 'en',
+        backend: {
+            loadPath: '/i18n/{{lng}}.json',
+        },
+        interpolation: {
+            escapeValue: false
+        }
+    });
+
+export default i18n;