Feature request: --sort option for rednote search (latest / popular)

## Feature request

Hi! Love the tool — it's been really useful for automating content discovery from Xiaohongshu.

### What I'd like

A `--sort` option for the `rednote search` command, mirroring the **综合** / **最新** tabs visible in the XHS search UI:

\`\`\`
opencli rednote search --sort latest
opencli rednote search --sort popular # current default behaviour
\`\`\`

### Why it matters

When searching by hashtag, the default relevance sort surfaces established high-engagement posts. New posts from smaller accounts — which are often more timely event announcements — don't appear in the first page of results even though they carry the hashtag.

Switching to **最新** sort in the XHS app surfaces those posts immediately. For any automation use case that needs fresh content (event discovery, news monitoring, etc.), this would be very valuable.

### What I think it involves

The XHS search page already renders a "最新" tab. I'd imagine it's a single click/navigation in the browser automation before results are collected — so hopefully a small change on your end.

Thanks for considering it!

Start at the `rednote search` command entry point and trace how it collects results from the XHS search page. Add the `--sort` handling for `latest` and `popular`, then verify that the selected sort matches the corresponding search UI tab and that the existing default behavior remains unchanged.

Written by the indexing model from the issue text.