feat(website): add QQ group QR code to navigation bar (#572)

Add a "QQ Group" tab in the site header that opens a centered modal
with the QR code image. Includes mobile menu support and i18n.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

packages/website/src/i18n/en.ts

@@ -3,6 +3,7 @@ export default {
     home: 'Home',
     docs: 'Documentation',
     github: 'GitHub',
+    qqGroup: 'QQ Group',
   },
   hero: {
     title: 'Self-Hosted AI Chat Dashboard',

packages/website/src/i18n/zh.ts

@@ -3,6 +3,7 @@ export default {
     home: '首页',
     docs: '文档',
     github: 'GitHub',
+    qqGroup: 'QQ 群',
   },
   hero: {
     title: '自托管 AI 聊天仪表板',