Add a QQ group button on the landing page #131

Problem / goal

Visitors on the main page have no way to join the project QQ group or copy its number. The group should sit in the header, to the right of the GitHub link.

Scope

• A Q群 button in the top-right of the landing page.
• A panel with the group name, the number 947508787, a link to join, and a copy action.
• Strings for all nine UI languages.

Acceptance

• The button is immediately to the right of the GitHub link.
• Opening it shows 群号 947508787 and the group name WebToApp 网页版.
• Join opens https://qm.qq.com/q/F9BHHJNvtm.
• Copy puts 947508787 on the clipboard, and a failed copy tells the visitor to select the number.