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/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: [],
 }
-