feat(overlay): add position customization for overlay placement

- Introduce PositionX and PositionY fields in OverlaySettings to allow manual positioning of the overlay.
- Update validation to ensure PositionX and PositionY are not less than -1.
- Modify diskConfig to include new position fields for JSON serialization.
- Implement functionality to save overlay position during drag events in the GUI.
- Enhance overlay placement logic to respect user-defined positions while ensuring proper display behavior.

src/gui/app.go

@@ -143,13 +143,16 @@ func (a *Application) activate() {
 	a.buildWindow()
 	a.installActions()
 	a.subscribeModels()
+	overlay.SetPositionSaver(a.saveOverlayPosition)
 	if a.launch.Warning != nil {
 		a.log.Add(diagnostics.Warning, "configuration", fmt.Sprintf("%v; using built-in defaults", a.launch.Warning))
 	}
+	// Show the main window before creating the overlay so a Layer Shell /
+	// display failure cannot leave the user with a silent no-GUI launch.
+	a.window.Present()
 	if err := a.configureOutputs(a.config, true); err != nil {
 		a.log.Add(diagnostics.Error, "outputs", err.Error())
 	}
-	a.window.Present()
 	a.applyAlwaysOnTop(a.config.GUI.MainWindowAlwaysOnTop)
 	if a.config.GUI.StartCaptions {
 		a.startCaptions()
@@ -395,6 +398,18 @@ func (a *Application) restartCaptions() {
 	}()
 }
 
+func (a *Application) saveOverlayPosition(x, y int) {
+	a.config.Overlay.PositionX = x
+	a.config.Overlay.PositionY = y
+	a.applied.Overlay.PositionX = x
+	a.applied.Overlay.PositionY = y
+	if err := config.Save(a.launch.Path, a.config); err != nil {
+		a.log.Add(diagnostics.Error, "configuration", fmt.Sprintf("Save overlay position: %v", err))
+		return
+	}
+	a.log.Add(diagnostics.Info, "overlay", fmt.Sprintf("Saved caption position (%d, %d)", x, y))
+}
+
 func (a *Application) configureOutputs(next config.AppConfig, initial bool) error {
 	var outputErr error
 	a.history.SetEnabled(next.Outputs.History)