feat: complete mobile UI implementation

project/system_option_ui_test.gd

@@ -104,6 +104,16 @@ func _run() -> void:
 	so.close()
 	_ck(not so.is_open(), "system-option window closed")
 
+	# 移动端专用横屏设置页：滑杆与单选项不依赖 PC 固定坐标小控件。
+	ui.set_mobile_mode(true)
+	so.set_mobile_mode(true)
+	so.open()
+	_ck(so.is_open() and so._mobile_root.size == Vector2(680, 340),
+			"mobile system-option window uses landscape base size")
+	_ck(so._mobile_root.find_children("*", "HSlider", true, false).size() == 2,
+			"mobile system-option exposes music and sound sliders")
+	so.close()
+
 	# --- 두 번째 인스턴스가 cfg 를 읽어 즉시 적용 ---
 	var au2 := FakeAudio.new()
 	var cam2 := FakeCam.new()