feat: 完成T-204b采集自动回写

采集完成且本轮有成功采集任务时，Tab① 自动启动 WriteBackWorker 回写旧标题和旧封面路径到当前批次 Excel。

保留手动回写旧数据到 Excel 作为自动回写失败后的重试入口；原 Excel 被占用时提示关闭文件后手动重试，SQLite 采集结果不回滚。

新增 GUI 测试覆盖自动触发、无成功采集不触发、锁文件提示手动重试；同步 T-204b 任务状态、current-state、api、architecture 与 progress。

tests/test_gui.py

@@ -387,6 +387,72 @@ class GuiTests(TempDirMixin, unittest.TestCase):
 
         self.assert_removed(temp_dir)
 
+    def test_collect_tab_auto_starts_write_back_after_collect_success(self):
+        with self.make_temp_dir() as temp_dir:
+            cfg = self.make_config(temp_dir)
+            db.init_db(cfg["db_path"])
+            batch_id = db.create_batch(["input.xlsx"], path=cfg["db_path"])
+            db.insert_tasks(
+                batch_id,
+                [
+                    {
+                        "source_file_abs": os.path.join(temp_dir, "input.xlsx"),
+                        "source_sheet": "商品",
+                        "source_row": 2,
+                        "account_name": "Excel主店",
+                        "alias": "alias-a",
+                        "item_id": "51100639510",
+                    }
+                ],
+                path=cfg["db_path"],
+            )
+            statuses = []
+            tab = CollectTab(config=cfg, status_callback=statuses.append)
+            self.addCleanup(tab.close)
+
+            with mock.patch.object(tab, "_start_write_back", return_value=True) as start_write_back:
+                tab._on_collect_finished({"collected": 1, "skipped": 0, "failed": 0})
+
+            start_write_back.assert_called_once_with(batch_id, auto=True)
+            self.assertIn("正在自动回写 Excel", statuses[-1])
+
+        self.assert_removed(temp_dir)
+
+    def test_collect_tab_does_not_auto_write_back_when_nothing_collected(self):
+        with self.make_temp_dir() as temp_dir:
+            cfg = self.make_config(temp_dir)
+            statuses = []
+            tab = CollectTab(config=cfg, status_callback=statuses.append)
+            self.addCleanup(tab.close)
+
+            with mock.patch.object(tab, "_start_write_back") as start_write_back:
+                tab._on_collect_finished({"collected": 0, "skipped": 1, "failed": 0})
+
+            start_write_back.assert_not_called()
+            self.assertEqual("采集完成：成功0，略过1，失败0", statuses[-1])
+
+        self.assert_removed(temp_dir)
+
+    def test_auto_write_back_locked_file_message_points_to_manual_retry(self):
+        with self.make_temp_dir() as temp_dir:
+            statuses = []
+            tab = CollectTab(config=self.make_config(temp_dir), status_callback=statuses.append)
+            self.addCleanup(tab.close)
+
+            with mock.patch("app.gui.QMessageBox.warning") as warning:
+                tab._on_write_back_failed(
+                    -1,
+                    "Excel 文件被占用，请关闭后重试: input.xlsx",
+                    auto=True,
+                )
+
+            message = warning.call_args[0][2]
+            self.assertIn("Excel 自动回写失败", message)
+            self.assertIn("点击「回写旧数据到 Excel」手动重试", message)
+            self.assertIn("手动重试", statuses[-1])
+
+        self.assert_removed(temp_dir)
+
     def test_write_back_worker_calls_excel_write_back(self):
         with mock.patch(
             "app.gui.excel.write_back",