feat: Upgrade to Tailwind CSS v4.1.18

- Migrate from tailwindcss v3.3 to v4.1.18
- Replace @tailwind directives with @import 'tailwindcss'
- Move custom colors to @theme block in globals.css
- Convert custom utilities to @utility syntax
- Update PostCSS config to use @tailwindcss/postcss
- Remove autoprefixer (now built-in)

package.json

@@ -44,6 +44,7 @@
   },
   "devDependencies": {
     "@playwright/test": "^1.57.0",
+    "@tailwindcss/postcss": "^4.1.18",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.1",
     "@types/node": "^20",
@@ -51,13 +52,12 @@
     "@types/react-dom": "^19.0.0",
     "@types/uuid": "^10.0.0",
     "@vitejs/plugin-react": "^5.1.2",
-    "autoprefixer": "^10.0.1",
     "eslint": "^8",
     "eslint-config-next": "16.1.0",
     "eslint-plugin-security": "^2.1.1",
     "jsdom": "^27.3.0",
     "postcss": "^8",
-    "tailwindcss": "^3.3.0",
+    "tailwindcss": "^4.1.18",
     "typescript": "^5",
     "vitest": "^4.0.16"
   },