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"

frontend/postcss.config.js

@@ -1,6 +1,6 @@
 export default {
   plugins: {
-    '@tailwindcss/postcss': {},
+    tailwindcss: {},
     autoprefixer: {},
   },
 }

frontend/tailwind.config.js

@@ -5,8 +5,11 @@ export default {
     "./src/**/*.{js,ts,jsx,tsx}",
   ],
   theme: {
-    extend: {},
+    extend: {
+      fontFamily: {
+        sans: ['Inter', 'system-ui', 'sans-serif'],
+      },
+    },
   },
   plugins: [],
 }
-