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/routes/app/add-product.tsx

@@ -0,0 +1,9 @@
+import { createFileRoute } from '@tanstack/react-router'
+
+export const Route = createFileRoute('/app/add-product')({
+  component: RouteComponent,
+})
+
+function RouteComponent() {
+  return <div>Hello "/app/add-product"!</div>
+}

frontend/src/routes/app/inventory.tsx

@@ -0,0 +1,17 @@
+import { createFileRoute, redirect } from "@tanstack/react-router";
+import { isAuthenticated } from "../../utils/auth";
+
+export const Route = createFileRoute("/app/inventory")({
+  beforeLoad: () => {
+    if (!isAuthenticated()) {
+      throw redirect({
+        to: "/login",
+      });
+    }
+  },
+  component: RouteComponent,
+});
+
+function RouteComponent() {
+  return <div>Hello "/app/inventory"!</div>;
+}

frontend/src/routes/app/view-product.tsx

@@ -0,0 +1,9 @@
+import { createFileRoute } from '@tanstack/react-router'
+
+export const Route = createFileRoute('/app/view-product')({
+  component: RouteComponent,
+})
+
+function RouteComponent() {
+  return <div>Hello "/app/view-product"!</div>
+}