feat: 完成T-204旧字段回写

实现 Excel 旧标题与旧封面路径回写原文件，按 source_file_abs/source_sheet/source_row 定位行，支持输出列自动追加、xlsm 保留 VBA、文件占用中文错误提示与 export_copy 另存副本。

Tab① 增加回写旧数据按钮与 WriteBackWorker，后台调用 excel.write_back，锁文件时提示关闭后重试；补充 Excel 与 GUI 单测覆盖回写、锁文件、另存副本和 worker 调用。

同步 harness 文档：T-204 标记完成，新增 T-204b 记录采集完成自动回写缺口，扩展 T-205 覆盖 Chrome 未启动/未登录引导保护，并更新 current-state、routes、api、architecture、requirements 与 progress。

tests/test_gui.py

@@ -24,6 +24,7 @@ from app.gui import (
     MainWindow,
     TAB_STYLE,
     TAB_TITLES,
+    WriteBackWorker,
 )
 
 
@@ -57,6 +58,10 @@ class GuiTests(TempDirMixin, unittest.TestCase):
             self.assertIn("padding: 8px 18px", window.tabs.styleSheet())
             self.assertIn("margin-right: 8px", window.tabs.styleSheet())
             self.assertIsInstance(window.tabs.widget(0), CollectTab)
+            self.assertEqual(
+                "回写旧数据到 Excel",
+                window.tabs.widget(0).write_back_button.text(),
+            )
 
         self.assert_removed(temp_dir)
 
@@ -382,6 +387,20 @@ class GuiTests(TempDirMixin, unittest.TestCase):
 
         self.assert_removed(temp_dir)
 
+    def test_write_back_worker_calls_excel_write_back(self):
+        with mock.patch(
+            "app.gui.excel.write_back",
+            return_value={"ok": True, "batch_id": "batch-1", "files": 1, "rows": 2},
+        ) as write_back:
+            summary = WriteBackWorker("batch-1", db_path="db.sqlite").execute()
+
+        self.assertEqual({"ok": True, "batch_id": "batch-1", "files": 1, "rows": 2}, summary)
+        write_back.assert_called_once_with(
+            "batch-1",
+            excel_path=None,
+            path="db.sqlite",
+        )
+
 
 if __name__ == "__main__":
     unittest.main()