Add a Linux WebKitGTK backend for the in-app browser

[zrtr]

Summary

• Enable the right-panel browser on Linux by rendering WebKitGTK offscreen and blitting the page into GPUI.
• Document the WebKitGTK 4.1 / GTK 3 runtime packages in the Linux install and contributor setup notes.

Test plan

• On Linux, open a Browser tab in the right panel and load a URL (navigation, back/forward, reload, stop).
• Type in the address bar, submit with Enter, and confirm the page receives keyboard focus (not the omnibox reclaiming it).
• Click, scroll, and select text on the page; confirm DevTools still opens.
• On macOS/Windows, confirm the existing WKWebView / WebView2 browser is unchanged.
• Confirm a machine without webkit2gtk-4.1 still builds, and the tab shows the unavailable state if WebKit init fails.

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