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/auth.ts

@@ -0,0 +1,55 @@
+import { API_BASE } from "../config/api.config";
+import Cookies from "js-cookie";
+import { useTranslation } from "react-i18next";
+import { toast } from "react-toastify";
+import { redirect } from "@tanstack/react-router";
+
+const { t } = useTranslation();
+
+export async function isAuthenticated() {
+  const result = await fetch(`${API_BASE}/users/verify-token`, {
+    method: "POST",
+    headers: {
+      Authorization: `Bearer ${Cookies.get("token") || ""}`,
+      "Content-Type": "application/json",
+      Accept: "application/json",
+    },
+  });
+
+  if (result.status === 200) {
+    return true;
+  }
+
+  return false;
+}
+
+export async function signInUser(username: string, password: string) {
+  const result = await fetch(`${API_BASE}/users/login`, {
+    method: "POST",
+    headers: {
+      Authorization: `Bearer ${Cookies.get("token") || ""}`,
+      "Content-Type": "application/json",
+      Accept: "application/json",
+    },
+    body: JSON.stringify({ username, password }),
+  });
+
+  const response = await result.json();
+
+  if (result.status === 202) {
+    Cookies.set("token", response.token);
+    return true;
+  }
+
+  if (result.status !== 202) {
+    Cookies.remove("token");
+    toast.error(t(response.code));
+  }
+}
+
+export function signOutUser() {
+  Cookies.remove("token");
+  throw redirect({
+    to: "/login",
+  });
+}

frontend/src/utils/i18n/index.ts

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+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;