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/vite.config.ts

@@ -2,12 +2,14 @@ import { defineConfig } from "vite";
 import react, { reactCompilerPreset } from "@vitejs/plugin-react";
 import babel from "@rolldown/plugin-babel";
 import tailwindcss from "@tailwindcss/vite";
+import { TanStackRouterVite } from "@tanstack/router-vite-plugin";
 
 // https://vite.dev/config/
 export default defineConfig({
   plugins: [
     react(),
     tailwindcss(),
+    TanStackRouterVite(),
     babel({ presets: [reactCompilerPreset()] }),
   ],
 });