fix(creator-studio): 固定公众号式创作工具栏

- 顶部改为 fixed 创作工具栏：左侧返回/品牌/模式，中间文章标题与保存状态，
  右侧撤销/重做、保存草稿、预览、发布
- 新增工作台级撤销/重做快照栈，连续编辑合并记录；支持 Ctrl/Cmd+Z / Shift+Z，
  不覆盖 RichText 输入框自身的编辑历史
- 移动端收敛工具栏布局，保留撤销/重做、预览、发布
- 先保存草稿再请求审核/直接发布；请求使用服务端更新后的文章与版本 ID
- 固定工具栏以独立 chrome 层布局，正文避开顶部工具栏高度
- 补充固定工具栏、历史操作与键盘快捷键契约

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

assets/creator-studio/build/index.css

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