feat: 完成PySide6主窗口骨架

- 实现 MainWindow + 固定顺序五 Tab QTabWidget + 状态栏

- 新增 offscreen GUI 单元测试覆盖 Tab 顺序和状态栏切换

- 缺 PySide6 时提供明确启动错误，避免无声失败

- 更新任务看板、API 合约、当前状态和进度记录

tests/test_gui.py

@@ -0,0 +1,45 @@
+import unittest
+import os
+import sys
+
+os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
+sys.path.insert(0, os.path.dirname(__file__))
+
+from app import gui
+
+if gui.QT_IMPORT_ERROR is not None:
+    raise unittest.SkipTest("PySide6 未安装")
+
+from PySide6.QtWidgets import QApplication
+
+from app.gui import MainWindow, TAB_TITLES
+
+
+class GuiTests(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        cls.app = QApplication.instance() or QApplication([])
+
+    def test_main_window_has_five_tabs_in_workflow_order(self):
+        window = MainWindow()
+        self.addCleanup(window.close)
+
+        self.assertEqual(5, window.tabs.count())
+        self.assertEqual(
+            TAB_TITLES,
+            [window.tabs.tabText(index) for index in range(window.tabs.count())],
+        )
+        self.assertEqual("就绪", window.statusBar().currentMessage())
+
+    def test_tab_switch_updates_status_bar(self):
+        window = MainWindow()
+        self.addCleanup(window.close)
+
+        window.tabs.setCurrentIndex(2)
+
+        self.assertEqual("③ 更新shopee", window.tabs.tabText(window.tabs.currentIndex()))
+        self.assertEqual("当前：③ 更新shopee", window.statusBar().currentMessage())
+
+
+if __name__ == "__main__":
+    unittest.main()