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/main.jsx

@@ -1,6 +1,7 @@
 import { StrictMode } from 'react';
 import { createRoot } from 'react-dom/client';
 import App from './App.jsx';
+import './i18n';
 import './style.css';
 import '@mdi/font/css/materialdesignicons.css';