Floating browser preview has no way to open its page in the system browser

[jamesvillarrubia]

The floating browser preview can't send its page to the system browser. Its pill has two actions: open in the right panel, and close. The "Open in system browser" button lives in the right panel's address bar. To reach it from the floating preview, you dock the preview into the panel first, which closes the floating preview.

@juliusmarminge called the extra docking step "useful discoverability feedback" when closing #14315, and asked for a maintainer's approval of the direction before a PR. This discussion asks for that call.

#14315 tried one option: a third pill button that calls the same shell.openExternal as the panel's button, with the tab's current URL. It is disabled when the tab has no URL, it shows an error toast if the open fails, and it appears only on browser previews (device streams have no URL).

Questions for maintainers:

1. Is opening the page in the system browser from the floating preview worth supporting?
2. If yes, is a pill button the right place, or would a context menu or shortcut fit the pill's minimal design better?