chore: run Tailwind v4 upgrade tool (Phase 1)

- Upgraded tailwindcss v3 → v4.2.1, postcss plugin to @tailwindcss/postcss
- Deleted tailwind.config.js, migrated theme to CSS @theme block in index.css
- Replaced @tailwind directives with @import 'tailwindcss'
- Added @custom-variant dark, @utility blocks for custom utilities
- Updated class names across 128 files (shadow-sm → shadow-xs, etc.)
- Removed autoprefixer (built into v4)
- Added migration plan doc

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

frontend/src/components/ui/Skeleton.tsx

@@ -6,7 +6,7 @@ export function Skeleton({ className, ...props }: SkeletonProps) {
   return (
     <div
       className={cn(
-        'animate-pulse rounded-lg bg-[rgba(255,255,255,0.06)]',
+        'animate-pulse rounded-lg bg-brand-border',
         className
       )}
       {...props}