feat(i18n): add internationalization support with English and German translations

- Integrated i18next for language detection and translation management.
- Added translation files for English and German languages.
- Implemented translation keys for notifications, reminders, and common UI elements.
- Updated main application entry point to include i18n initialization.
- Styled language selection dropdown in settings.
- Enhanced package dependencies to include i18next and react-i18next.

frontend/src/main.tsx

@@ -3,6 +3,7 @@ import ReactDOM from "react-dom/client";
 import { BrowserRouter } from "react-router-dom";
 import App from "./App";
 import "./styles.css";
+import "./i18n";
 
 ReactDOM.createRoot(document.getElementById("root")!).render(
   <React.StrictMode>