Update WeChat group QR code — 2026-09-25

[github-actions]

1. Paste the new WeChat group QR image directly into this issue.

2. Give the agent this prompt:

Find the open issue whose title contains `qr` (case-insensitive) with `gh issue list --state open --json number,title`, fetch the QR image from that issue, run `git pull --ff-only`, replace `assets/wechat-group-qr.jpg` with the image, commit the change, and push it to `main`.