feat: integrate TanStack Router and update routing structure

- Added TanStack Router for improved routing management.
- Created route tree and individual routes for login, index, add-product, inventory, and view-product.
- Implemented authentication checks for the inventory route.
- Introduced a landing page component.
- Updated App component to utilize RouterProvider and ToastContainer for notifications.
- Refactored CSS to use Tailwind CSS, removing custom styles.
- Added API configuration for backend URL management.
- Implemented authentication utilities for user sign-in and sign-out.
- Integrated i18next for internationalization with English and German language support.
- Created localization files for English and German languages.

frontend/src/utils/i18n/index.ts

@@ -0,0 +1,34 @@
+import i18n from "i18next";
+import { initReactI18next } from "react-i18next";
+import Cookies from "js-cookie";
+
+import enLang from "./locales/en/en.json";
+import deLang from "./locales/de/de.json";
+
+// the translations
+// (tip move them in a JSON file and import them,
+// or even better, manage them separated from your code: https://react.i18next.com/guides/multiple-translation-files)
+const resources = {
+  en: {
+    translation: enLang,
+  },
+  de: {
+    translation: deLang,
+  },
+};
+
+i18n
+  .use(initReactI18next) // passes i18n down to react-i18next
+  .init({
+    resources,
+    fallbackLng: "en", // use en if detected lng is not available
+    lng: Cookies.get("language") || "en", // language to use, more information here: https://www.i18next.com/overview/configuration-options#languages-namespaces-resources
+    // you can use the i18n.changeLanguage function to change the language manually: https://www.i18next.com/overview/api#changelanguage
+    // if you're using a language detector, do not define the lng option
+
+    interpolation: {
+      escapeValue: false, // react already safes from xss
+    },
+  });
+
+export default i18n;