Add -reload-theme CLI flag to re-apply the current theme file (#860)

* Add -reload-theme CLI flag to re-apply theme via IPC

Sends a reload-theme IPC message to a running instance, causing it to
call fyne.CurrentApp().Settings().SetTheme(m.theme). Follows the same
callback pattern as -show (OnReactivate) and quit (OnExit).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* actually re-read theme file

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app.go

@@ -60,6 +60,7 @@ type App struct {
 	// UI callbacks to be set in main
 	OnReactivate func()
 	OnExit       func()
+	OnReloadTheme func()
 
 	appName        string
 	displayAppName string
@@ -227,7 +228,8 @@ func StartupApp(appName, displayAppName, appVersion, appVersionTag, latestReleas
 				ipcRatingHandler,
 				a.ServerManager,
 				a.callOnReactivate,
-				func() { _ = a.callOnExit() })
+				func() { _ = a.callOnExit() },
+				a.callOnReloadTheme)
 			go a.ipcServer.Serve(listener)
 		} else {
 			log.Printf("error starting IPC server: %s", err.Error())
@@ -336,6 +338,12 @@ func (a *App) callOnReactivate() {
 	}
 }
 
+func (a *App) callOnReloadTheme() {
+	if a.OnReloadTheme != nil {
+		a.OnReloadTheme()
+	}
+}
+
 func (a *App) callOnExit() error {
 	if a.OnExit == nil {
 		return errors.New("no quit handler registered")
@@ -658,6 +666,8 @@ func (a *App) checkFlagsAndSendIPCMsg(cli *ipc.Client) error {
 		return cli.PauseAfterCurrent()
 	case *FlagShow:
 		return cli.Show()
+	case *FlagReloadTheme:
+		return cli.ReloadTheme()
 	case VolumeCLIArg >= 0:
 		return cli.SetVolume(VolumeCLIArg)
 	case VolumePctCLIArg != 0: