About: add "Contributor" button with live contributor network

[BenJule]

Add a Contributor button next to the existing Portions copyright (License Info) button in the About dialog.

Clicking it opens a modal that embeds the BambuStudio contributor network in a WebView:

• Live data — the graph is fetched live from the GitHub contributors API on open.
• Offline handling — if there is no connection, the page shows a clear message asking the user to connect to the internet, with a Retry button.
• Theme — Light/Dark is controlled by BambuStudio (passed via a ?theme= URL parameter, consistent with the theme selector in feat(gui): add dark mode theme selector (Light / Dark / Follow system) bambulab/BambuStudio#11241); the page has no theme switch of its own.

The visualisation is a radial constellation: BambuStudio at the core, contributors on the ring sized by commit count, and the other open-source projects they share.