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>

main.go

@@ -110,6 +110,7 @@ func main() {
 	mainWindow.Window.SetMaster()
 	myApp.OnReactivate = util.FyneDoFunc(mainWindow.Show)
 	myApp.OnExit = util.FyneDoFunc(mainWindow.Quit)
+	myApp.OnReloadTheme = util.FyneDoFunc(mainWindow.ReloadTheme)
 
 	if runtime.GOOS == "windows" {
 		windowStartupTasks := sync.OnceFunc(func() {