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.

backend/routes/app/users.route.js

@@ -1,10 +1,14 @@
 import express from "express";
 import dotenv from "dotenv";
-import { generateToken } from "../../services/tokenService.js";
+import { authenticate, generateToken } from "../../services/tokenService.js";
 import { findUser, loginUser } from "./database/users.database.js";
 dotenv.config();
 const router = express.Router();
 
+router.post("/verify-token", authenticate, async (req, res) => {
+  res.status(200);
+});
+
 router.post("/login", async (req, res) => {
   const username = req.body.username;
   const password = req.body.password;