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/client.go

@@ -128,6 +128,11 @@ func (c *Client) Show() error {
 	return err
 }
 
+func (c *Client) ReloadTheme() error {
+	_, err := c.sendRequest(ReloadThemePath)
+	return err
+}
+
 func (c *Client) Quit() error {
 	_, err := c.sendRequest(QuitPath)
 	return err