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/package.json

@@ -18,24 +18,28 @@
     "@emotion/styled": "^11.14.1",
     "@fontsource/inter": "^5.2.8",
     "@mui/joy": "^5.0.0-beta.52",
+    "@tailwindcss/vite": "^4.3.0",
     "@tanstack/react-form": "^1.32.0",
     "@tanstack/react-query": "^5.100.14",
     "@tanstack/react-router": "^1.170.8",
-    "react": "^19.2.6",
-    "react-dom": "^19.2.6",
-    "@tailwindcss/vite": "^4.3.0",
     "i18next": "^26.0.10",
-    "js-cookie": "^3.0.5",
+    "js-cookie": "^3.0.7",
+    "lucide-react": "^1.14.0",
+    "react": "^17.0.0 || ^18.0.0 || ^19.0.0",
+    "react-dom": "^17.0.0 || ^18.0.0 || ^19.0.0",
+    "react-i18next": "^17.0.8",
+    "react-toastify": "^11.1.0",
     "tailwindcss": "^4.3.0",
     "validator": "^13.15.35",
-    "zod": "^4.4.3",
-    "lucide-react": "^1.14.0"
+    "zod": "^4.4.3"
   },
   "devDependencies": {
     "@babel/core": "^7.29.0",
     "@eslint/js": "^10.0.1",
     "@rolldown/plugin-babel": "^0.2.3",
+    "@tanstack/router-vite-plugin": "^1.167.11",
     "@types/babel__core": "^7.20.5",
+    "@types/js-cookie": "^3.0.6",
     "@types/node": "^24.12.3",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
@@ -49,4 +53,4 @@
     "typescript-eslint": "^8.59.2",
     "vite": "^8.0.12"
   }
-}
+}