fix: add tray Quit for Windows/MacOS and exit-on-close for Linux

[ZS041]

closes #146

Linux gets neither a system-provided menu bar (GTK) nor a reliably visible tray icon (default GNOME sessions often don't show AppIndicator tray icons without the AppIndicator extension installed), leaving users with no way to quit the app short of killing the process. macOS has Cmd+Q via NSApp, Windows has the automatic Window menu.

• add a "Quit Datum" item to the tray menu (works everywhere)
• on Linux, change WindowCloseBehaviour to WindowCloses so the close button actually exits the app; macOS/Windows keep WindowHides so the tray flow still works as a menu-bar app

happy to make changes as suggested, I'm not sure if the Linux exit on close is what we actually want given the conversation between @privateip and @zachsmith1, or if a more creative solution is in order

[ZS041]

After digging into this a bit more, I think we can probe the session bus at startup for a StatusNotifier tray host and adjust behavior accordingly.

That would let us avoid assuming tray support exists everywhere. Iff a host is available, minimizing to tray makes sense, if not, we can fall back to the current close/quit behavior for Linux.

Another improvement would be showing a quick toast/notification the first time the app is minimized to tray. That would address @privateip’s concern that there’s currently no indication the app is still running after the window closes.

[privateip]

@ZS041 that feels like a good middle of the road solution. I'm far from a UI person so I'm not sure what would be considered best practices here but that seems like a logical iteration