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/ipc/api.go

@@ -26,6 +26,7 @@ const (
 	VolumePath            = "/volume"            // ?v=<vol>
 	VolumeAdjustPath      = "/volume/adjust"     // ?pct=<+/- percentage>
 	ShowPath              = "/window/show"
+	ReloadThemePath       = "/window/reload-theme"
 	QuitPath              = "/window/quit"
 	RateCurrentTrackPath  = "/current_track/rate" // ?r=<rating 0-5>
 )