fix: don't show the tray window at startup with --minimized - #8

Starting Coder Desktop with --minimized (which the XDG autostart entry does) leaves the tray window on screen as an empty, always-on-top rectangle. With KWin's blur enabled it's a blurred box in the middle of a monitor, and it only goes away once you open and close the tray.

The tray window was the lifetime's MainWindow, which Avalonia always shows at startup, so --minimized just hid it again straight away. That hide lands before the window manager has mapped the window, so it does nothing and the window gets mapped anyway, whilst Avalonia thinks it's hidden and never draws into it.

To fix, we no longer make the tray window the MainWindow, so it's only shown on a normal launch or when the tray icon is clicked.