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/editor-ai/EditorAIPanel.tsx

@@ -65,7 +65,7 @@ export function EditorAIPanel({
         </div>
         <button
           onClick={onClose}
-          className="p-1.5 rounded-lg hover:bg-[rgba(255,255,255,0.06)] text-muted-foreground hover:text-foreground transition-colors"
+          className="p-1.5 rounded-lg hover:bg-brand-border text-muted-foreground hover:text-foreground transition-colors"
         >
           <X size={16} />
         </button>