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>

postcss.config.cjs

@@ -1,6 +1,5 @@
 module.exports = {
   plugins: {
-    tailwindcss: {},
-    autoprefixer: {},
+    "@tailwindcss/postcss": {},
   },
 };