feat(web): add QQ beta-community group to homepage + READMEs

Fills the long-empty "内 测 用 户 群" placeholder (was "群二维码 /
邀请链接（待补充）") on the homepage contact grid with the real QQ
group QR (group ID 575404333) plus a scan-or-search line.

Mirrors it across all three READMEs as a scan-to-join block right
after the contact line, rendered from apps/web/public/qq-group.webp
(760×760 QR-only crop with a white quiet zone, ~45KB).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -56,6 +56,10 @@ The project is still very early and many features are far from polished. We'd lo
 
 Get in touch: hi@infiplot.com
 
+Scan to join our **beta community on QQ** (group ID `575404333`) to share feedback and help shape the project:
+
+<img src="apps/web/public/qq-group.webp" alt="InfiPlot beta community QQ group QR code" width="200" />
+
 ---
 
 ## How it works