- Add browser_automation.py: HTTP→Headless Browser adapter
- 6 platform default browser scripts (douyin/bilibili/kuaishou/xiaohongshu/toutiao/channels)
- {{variable}} substitution for content fields
- configurable browser_endpoint in extra_config
- engine.py: route browser-only platforms to browser_execute()
- platfoms.py: remove NotImplementedError stubs