feat(mindspace): 全部页面管理、级联删除与页面同步去重

支持全部页面分页/多选/删除/分享，删除时可选移除广场帖并清理工作区附件；修复并发同步重复创建页面，并补齐预览下载链接重写与 iframe 下载权限。

Co-authored-by: Cursor <cursoragent@cursor.com>

src/components/ChatSharePreviewModal.tsx

@@ -73,7 +73,7 @@ export function ChatSharePreviewModal({
         <button type="button" className="chat-share-preview-close" onClick={onClose} aria-label="关闭">✕</button>
       </div>
       <div className="chat-share-preview-body">
-        <iframe src={previewUrl} title="页面预览" sandbox="allow-same-origin allow-scripts allow-popups" className="chat-share-preview-frame" />
+        <iframe src={previewUrl} title="页面预览" sandbox="allow-same-origin allow-scripts allow-popups allow-downloads" className="chat-share-preview-frame" />
 
         <div className="chat-share-side-actions">
           {phase.kind === 'done' ? (