Summary

Add Zhihu (知乎) platform support for article synchronization.

Changes

Browser Extension

• Implemented Zhihu-specific sync logic in syncToPlatform() using the "Import Document" feature
• The extension navigates to https://zhuanlan.zhihu.com/write, fills the title, and uploads the markdown content as a .md file

Implementation Details

Instead of direct content pasting (which fails on Zhihu), this PR uses Zhihu's native "Import Document" feature:

1. Open Zhihu writing page
2. Fill article title
3. Click "导入" (Import) button → Click "导入文档" (Import Document) submenu
4. Programmatically create and upload a .md file using the File API

Technical Notes

• Button selectors use innerText.includes() to handle zero-width characters in Zhihu's UI
• File is created in memory using new File() API with text/plain MIME type
• Both change event and drag-drop fallback are triggered for compatibility

Testing

1. Install the browser extension from cose/ directory
2. Open the editor and select Zhihu for sync
3. Verify the article title and content are imported correctly