fix: 添加 GitHub Actions post-build 步驟

- 修正 GitHub Pages 根目錄 404 問題
- 確保 scripts/post-build.js 在部署時執行
- 自動生成根 index.html 語言選擇頁面

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: peoplewhite

.github/workflows/deploy.yml

@@ -34,6 +34,9 @@ jobs:
       - name: Build with Next.js
         run: npm run build
 
+      - name: Post-build setup
+        run: node scripts/post-build.js
+        
       - name: Upload artifact
         uses: actions/upload-pages-artifact@v3
         with: