Publish builds every desktop target selected in project settings

Publishing a desktop app should build every build target ticked in project settings, regardless of which operating system Studio is running on.

Today, Publish silently skips desktop targets that belong to another OS unless the user also finds and ticks an "advanced cross-host output" checkbox in the Publish dialog. A user on macOS who selects Apple Silicon, Windows and Linux in the project inspector expects all three builds and instead gets only the Mac build, with no post-publish explanation.

With this change, the selected targets are the only gate:

• Publish builds every selected desktop target on every host.
• On a Windows host, the Windows format still defaults to MSIX. On other hosts it defaults to a portable ZIP, which is a finished, runnable Windows app that the current host can produce completely. MSIX can still be selected off Windows and then produces the existing ready-to-package handoff folder, archive, and instructions.
• Linux publishing gains a portable package option (a .tar.gz containing the complete runnable app), so a finished Linux build can be produced on any host. AppImage remains the default format on Linux hosts and can still be selected elsewhere, where it produces the existing AppDir handoff.