[MM-61864] Fix issue where the app will not restore when opened again from cold, fix issue where deep linking from cold didn't work on Linux

[devinbinnie]

Summary

After fixing https://github.com/mattermost/desktop/pull/3201, I introduced a bug where the app would not restore its window when booting a second instance, due to the removal of MainWindow.show(), which was removed since it interfered with the normal app boot process from cold. I also noticed that from cold, Linux would ignore deep links.

This PR fixes both of the above issues.

Ticket Link

https://mattermost.atlassian.net/browse/MM-61864

Fix issue where the app will not restore when opened again from cold
Fix issue where deep linking from cold didn't work on Linux

[mattermost-build]

Cherry pick is scheduled.