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>

ui/mainwindow.go

@@ -273,6 +273,11 @@ func (m *MainWindow) setInitialSize() {
 	m.Window.Resize(m.DesiredSize())
 }
 
+func (m *MainWindow) ReloadTheme() {
+	m.theme.ReloadThemeFile()
+	fyne.CurrentApp().Settings().SetTheme(m.theme)
+}
+
 func (m *MainWindow) StartupPage() controller.Route {
 	switch m.App.Config.Application.StartupPage {
 	case "Artists":