Add preference to keep the main window hidden during shortcut-triggered sessions

[zugaldia]

Currently, when the user triggers the keyboard shortcut, the main application window appears to start recording. A second shortcut press stops recording, runs transcription (and optional LLM polish), and then hides the window using visible = false — a deliberate choice to restore it on the current workspace when relaunched. This works well, but users who rely on the shortcut exclusively may prefer the window never appear at all.

This ticket proposes a new preference (name TBD) that suppresses the main window during shortcut-triggered sessions. When enabled, the window would be minimized rather than hidden, so users can still access it through the taskbar or launcher if needed. The recording flow itself would proceed entirely in the background, which is reasonable since GNOME already provides visual feedback: the microphone indicator appears in the top bar during recording, and the Remote Desktop session indicator is also visible.

[zugaldia]

This mechanism would also require integrating notifications so that if anything goes wrong while the main window is hidden, we would use the desktop notifications to tell the user what is that didn't work, rather than failing silently. This would require adding support for https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.Notification.html to https://github.com/zugaldia/stargate.

[zugaldia]

Partially addressed by #66. We did integrate a new boolean preference to keep the main window hidden as opt-in. The ability to show errors in a notification or in an extension #15 is still pending.

[zugaldia]

The ability to show desktop notifications has now been addressed with #77.