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.

public/i18n/de.json

@@ -0,0 +1,5 @@
+{
+  "welcome": "Dein Game-Backlog unter Kontrolle",
+  "login": "Anmelden",
+  "imprint": "Impressum"
+}

public/i18n/en.json

@@ -0,0 +1,5 @@
+{
+  "welcome": "Control your game backlog",
+  "login": "Login",
+  "imprint": "Imprint"
+}