fix: downgrade to Tailwind v3 for stable CSS compilation

- Remove Tailwind v4 which has breaking changes
- Install Tailwind v3.4.0 (stable)
- Update PostCSS config for v3
- Ensure proper CSS compilation

frontend/package.json

@@ -15,7 +15,6 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.39.1",
-    "@tailwindcss/postcss": "^4.1.17",
     "@types/node": "^24.10.1",
     "@types/react": "^19.2.5",
     "@types/react-dom": "^19.2.3",
@@ -26,7 +25,7 @@
     "eslint-plugin-react-refresh": "^0.4.24",
     "globals": "^16.5.0",
     "postcss": "^8.5.6",
-    "tailwindcss": "^4.1.17",
+    "tailwindcss": "^3.4.18",
     "typescript": "~5.9.3",
     "typescript-eslint": "^8.46.4",
     "vite": "^7.2.4"