feat: add a QQ group button beside GitHub

[shiaho777]

Summary

• Add a Q群 button to the right of the GitHub link on the landing page.
• The panel shows the group name WebToApp 网页版, the number 947508787, a join link, and a copy action.

Fixes

Fixes #131

Test plan

• Open the landing page and confirm Q群 sits to the right of the GitHub icon.
• Open the panel and confirm the number is 947508787 and the join link is https://qm.qq.com/q/F9BHHJNvtm.
• Copy the number, then close the panel with Escape and by clicking outside.
• Check a phone-width window so the button stays on screen.
• CI green (required check: test (3.11))

Notes for review

The join URL opens in a new tab. If the clipboard is blocked, the panel asks the visitor to select the number.

Made with Cursor