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/copilot/CopilotToggle.tsx

@@ -11,7 +11,7 @@ export function CopilotToggle({ isOpen, onToggle }: CopilotToggleProps) {
   return (
     <button
       onClick={onToggle}
-      className="fixed bottom-6 right-6 z-40 bg-gradient-brand text-[#101114] p-3.5 rounded-full shadow-lg shadow-primary/30 hover:opacity-90 active:scale-[0.97] transition-all"
+      className="fixed bottom-6 right-6 z-40 bg-gradient-brand text-brand-dark p-3.5 rounded-full shadow-lg shadow-primary/30 hover:opacity-90 active:scale-[0.97] transition-all"
       title="Open AI Copilot"
     >
       <MessageCircle size={22} />