Feature Request: Add option to declare note as original when posting

### Feature Request: Add option to declare note as original when posting

**Is your feature request related to a problem? Please describe.**
Currently, when posting a note via `xhs post`, there isn't an option to explicitly declare the content as "original". This is a common and important setting for creators on Xiaohongshu to protect their work and gain better visibility.

**Describe the solution you'd like.**
I would like to see a new option, for example, `--original` or `--declare-original`, added to the `xhs post` command. When this flag is used, the posted note should be marked as original on the Xiaohongshu platform.

**Describe alternatives you've considered.**
I have reviewed the `xhs_cli/commands/creator.py` and `xhs_cli/client_mixins.py` files, specifically the `post` command and the `create_image_note` method. It appears that the current API call to `/web_api/sns/v2/note` does not expose a parameter for declaring originality. The `privacy_info` and `source` fields do not seem to be related to this.

It's possible that:
1. The Xiaohongshu API does not support declaring originality via this specific endpoint, and it might be handled through a different API or a web/app UI interaction (e.g., a checkbox during manual posting).
2. The API does support it, but the relevant parameter hasn't been reverse-engineered or integrated into the CLI yet.

**Additional context**
We believe this would be a valuable feature for users who wish to publish original content and leverage the platform's support for such posts.

Could you please investigate if the underlying Xiaohongshu API for posting notes supports an "original" declaration, and if so, consider adding this functionality to `xiaohongshu-cli`? Your expertise in reverse-engineering the API would be greatly appreciated.

Thank you!

Start in xhs_cli/commands/creator.py at the xhs post command and in xhs_cli/client_mixins.py at create_image_note. Investigate whether the /web_api/sns/v2/note endpoint supports an originality parameter, using the existing API behavior and any relevant posting flow as the starting point. Done means determining whether the flag can be supported and, if so, defining its end-to-end posting behavior.

Written by the indexing model from the issue text.