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/tailwind.config.js

@@ -46,6 +46,20 @@ export default {
           800: '#1e293b',
           900: '#0f172a',
           950: '#020617'
+        },
+        // PURO AI 设计系统色板（给新页面 Landing/Auth/Docs 用，不影响 admin）
+        puro: {
+          cyan: '#22d3ee',
+          'cyan-2': '#67e8f9',
+          purple: '#a855f7',
+          amber: '#fbbf24',
+          green: '#34d399',
+          red: '#f87171',
+          // 平台品牌点色
+          claude: '#d97757',
+          gpt: '#10a37f',
+          gemini: '#4285f4',
+          codex: '#f0a030'
         }
       },
       fontFamily: {