Upgrade Tailwind CSS from 3 to 4 with CSS-first config migration

Migrate tailwind.config.js to @theme in CSS, replace PostCSS plugins
with @tailwindcss/postcss, add @reference to 12 Vue scoped styles using
@apply, and remove autoprefixer (now built into Tailwind 4).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -16,6 +16,7 @@
   },
   "dependencies": {
     "@primevue/themes": "^4.5.4",
+    "@tailwindcss/postcss": "^4.2.1",
     "axios": "^1.13.6",
     "chart.js": "^4.5.1",
     "chartjs-adapter-moment": "^1.0.1",
@@ -55,7 +56,6 @@
     "@vitejs/plugin-vue": "^5.2.4",
     "@vue/eslint-config-prettier": "^10.2.0",
     "@vue/test-utils": "^2.4.6",
-    "autoprefixer": "^10.4.27",
     "cypress": "^15.11.0",
     "cypress-xpath": "^2.0.1",
     "eslint": "^9.39.3",
@@ -66,7 +66,7 @@
     "prettier": "^3.8.1",
     "sass": "^1.97.3",
     "start-server-and-test": "^2.1.5",
-    "tailwindcss": "^3.4.19",
+    "tailwindcss": "^4.2.1",
     "ts-node": "^10.9.2",
     "typescript": "^5.9.3",
     "vite": "^6.4.1",