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>

vite.config.ts

@@ -1,5 +1,6 @@
 import { defineConfig } from "vite";
 import react from "@vitejs/plugin-react";
+import tailwindcss from "@tailwindcss/vite";
 import path from "node:path";
 import { fileURLToPath } from "node:url";
 
@@ -7,7 +8,7 @@ const __dirname = path.dirname(fileURLToPath(import.meta.url));
 
 // https://vite.dev/config/
 export default defineConfig({
-  plugins: [react()],
+  plugins: [react(), tailwindcss()],
   resolve: {
     alias: {
       "@": path.resolve(__dirname, "./src"),