Migrate from Tailwind CSS v3 to v4 (#219)

- Replace JS config (tailwind.config.js) with CSS-first @theme directive
- Add @tailwindcss/vite plugin for improved Vite integration
- Update src/index.css with v4 syntax (@import, @theme, @utility)
- Convert @layer utilities to @utility syntax
- Fix hardcoded scrollbar colors in command-launcher.css
- Add Tailwind v4 skill document (.claude/skills/tailwind-v4.md)
- Update CLAUDE.md with Tailwind v4 quick reference

https://claude.ai/code/session_01T6RenqDof8br6Nt9aKcjvq

Co-authored-by: Claude <noreply@anthropic.com>

src/components/command-launcher.css

@@ -192,9 +192,9 @@
 }
 
 .command-list::-webkit-scrollbar-thumb {
-  background-color: rgba(255, 255, 255, 0.2);
+  background-color: hsl(var(--scrollbar-thumb));
 }
 
 .command-list::-webkit-scrollbar-thumb:hover {
-  background-color: rgba(255, 255, 255, 0.3);
+  background-color: hsl(var(--scrollbar-thumb-hover));
 }