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>

src/views/Files/Files.vue

@@ -623,6 +623,7 @@
   }
 </script>
 <style scoped>
+@reference "../../assets/tailwind.css";
 :deep(thead) {
   @apply sticky top-0 bg-neutral-10 after:border-b after:border-neutral-500 after:w-full after:left-0 after:bottom-0 after:absolute table table-fixed w-full
 }