T1000E: blink LED on shutdown when buzzer is muted

On long-press shutdown with buzzer muted, flash the heartbeat LED 3
times as a visual power-off indicator. Auto-shutdown paths are unaffected.

zephcore/helpers/ui-button/ui_task.c

@@ -503,6 +503,12 @@ static void action_deep_sleep(void)
 		k_sleep(K_MSEC(50));
 	}
 	buzzer_stop();
+
+#ifdef CONFIG_BOARD_T1000_E
+	if (buzzer_is_quiet()) {
+		ui_led_flash_shutdown();
+	}
+#endif
 #endif
 
 	/* Shared peripheral teardown + SENSE config for sw0 wake.