chore(deps): update tailwindcss to v4

Switch to the @tailwindcss/vite plugin and the @import entry. Load the existing config with @config and keep class-based dark mode through a custom variant. Remove the postcss config and its autoprefixer dependency, which v4 no longer needs.

frontend/package.json

@@ -27,14 +27,13 @@
   "devDependencies": {
     "@fortawesome/fontawesome-free": "^7.1.0",
     "@tailwindcss/typography": "^0.5.19",
+    "@tailwindcss/vite": "^4.3.3",
     "@testing-library/jest-dom": "^7.0.0",
     "@testing-library/vue": "^8.1.0",
     "@vitejs/plugin-vue": "^6.0.3",
     "@vue/test-utils": "^2.4.6",
-    "autoprefixer": "^10.4.23",
     "jsdom": "^30.0.0",
-    "postcss": "^8.5.23",
-    "tailwindcss": "^3.4.19",
+    "tailwindcss": "^4.3.3",
     "typescript": "^6.0.3",
     "vite": "^8.0.0",
     "vitest": "^5.0.3",