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>

package.json

@@ -101,7 +101,7 @@
     "@types/ws": "^8.18.1",
     "@vitejs/plugin-react": "^4.3.4",
     "@vitest/ui": "^4.0.15",
-    "autoprefixer": "^10.4.20",
+    "@tailwindcss/vite": "^4.0.0",
     "eslint": "^9.17.0",
     "eslint-config-prettier": "^10.1.8",
     "eslint-plugin-prettier": "^5.5.4",
@@ -114,7 +114,7 @@
     "postcss": "^8.4.49",
     "prettier": "^3.7.4",
     "sharp": "^0.34.5",
-    "tailwindcss": "^3.4.17",
+    "tailwindcss": "^4.0.0",
     "typescript": "~5.6.2",
     "typescript-eslint": "^8.18.2",
     "vite": "^6.0.5",