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>

postcss.config.js

@@ -1,6 +1,5 @@
+// PostCSS config - minimal since we use @tailwindcss/vite plugin
+// This file can be removed if no other PostCSS plugins are needed
 export default {
-  plugins: {
-    tailwindcss: {},
-    autoprefixer: {},
-  },
+  plugins: {},
 };