feat(design): scaffold PURO AI design system

- Add puro.css (tokens + primitives) as global stylesheet
- Load Inter + JetBrains Mono via Google Fonts
- Extend tailwind.config with puro.* color namespace (no conflict with legacy primary/accent palettes)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/main.ts

@@ -5,6 +5,7 @@ import router from './router'
 import i18n, { initI18n } from './i18n'
 import { useAppStore } from '@/stores/app'
 import './style.css'
+import './assets/puro.css'
 
 function initThemeClass() {
   const savedTheme = localStorage.getItem('theme')