Only enable .onHover tracking on TitlebarControlButton when the style
uses hoverBackground (e.g. pillGroup). Styles without a visible hover
background no longer install the tracking area, preventing the crash
on notification-bell hover.
Also switches the notifications anchor from .overlay to .background so
AppKit hit-testing no longer conflicts with the popover anchor view.
Includes regression test for the hover-tracking policy.
Fixes https://github.com/manaflow-ai/cmux/issues/537
Fixes https://github.com/manaflow-ai/cmux/issues/180 by enabling NSAllowsArbitraryLoadsInWebContent for WKWebView and adding a regression test that asserts ATS web-content override exists in Resources/Info.plist.