fix(ui): install X11 error handler so tray races don't kill the UI

Xlib's default protocol-error handler calls exit() on any async X error,
so a tray race (tray manager window dying between find and dock, XMoveWindow
on a popup the WM already destroyed) would take the whole UI process down.

Install process-global logging no-op handlers via XSetErrorHandler/
XSetIOErrorHandler after every XOpenDisplay. Since the handler slot is
process-global and GDK init can clobber it, additionally wrap the popup
code's raw Xlib calls on GDK's Display in gdk_x11_display_error_trap_push/
pop_ignored, which is independent of the global handler.

client/ui/xembed_host_linux.go

@@ -73,6 +73,7 @@ func newXembedHost(conn *dbus.Conn, busName string, objPath dbus.ObjectPath) (*x
 	if dpy == nil {
 		return nil, errors.New("cannot open X display")
 	}
+	C.xembed_install_error_handlers()
 
 	screen := C.xembed_default_screen(dpy)
 	trayMgr := C.xembed_find_tray(dpy, screen)
@@ -383,6 +384,7 @@ func xembedTrayAvailable() bool {
 	if dpy == nil {
 		return false
 	}
+	C.xembed_install_error_handlers()
 	defer C.XCloseDisplay(dpy)
 	screen := C.xembed_default_screen(dpy)
 	return C.xembed_find_tray(dpy, screen) != 0