refactor(chain): _mixins.py 更名 _transfer.py，统一功能域 mixin 命名

TransferChain 的 7 个功能域 mixin 集合文件按域更名 _transfer.py，
与 _recognition/_messaging/_interaction/_music 的单域命名约定对齐；
同步更新 transfer.py 导入、9 个测试文件 patch 目标与架构文档。
另含 chain 基类导入排序优化与未使用导入清理（copy/datetime 已随
dispatch 方法迁入 _messaging.py）。

tests/test_transfer_mounted_disk_cleanup.py

@@ -31,7 +31,7 @@ def test_enabled_cleanup_skips_filesystem_detection():
     开关开启时应保持旧行为，且不产生额外文件系统检测。
     """
     with patch(
-            "app.chain._mixins.SystemUtils.is_network_filesystem"
+            "app.chain._transfer.SystemUtils.is_network_filesystem"
     ) as is_network_filesystem:
         should_delete = (
             TransferChain._should_delete_empty_source_directories(
@@ -50,7 +50,7 @@ def test_disabled_cleanup_keeps_mounted_local_source_directories():
     开关关闭时应保留网络或 FUSE 挂载的本地源目录。
     """
     with patch(
-            "app.chain._mixins.SystemUtils.is_network_filesystem",
+            "app.chain._transfer.SystemUtils.is_network_filesystem",
             return_value=True,
     ) as is_network_filesystem:
         should_delete = (
@@ -72,7 +72,7 @@ def test_disabled_cleanup_still_deletes_ordinary_local_source_directories():
     开关关闭时普通本地文件系统仍应删除空目录。
     """
     with patch(
-            "app.chain._mixins.SystemUtils.is_network_filesystem",
+            "app.chain._transfer.SystemUtils.is_network_filesystem",
             return_value=False,
     ):
         should_delete = (
@@ -91,7 +91,7 @@ def test_disabled_cleanup_does_not_change_remote_storage_cleanup():
     开关关闭时非本地存储仍应执行原有空目录清理。
     """
     with patch(
-            "app.chain._mixins.SystemUtils.is_network_filesystem"
+            "app.chain._transfer.SystemUtils.is_network_filesystem"
     ) as is_network_filesystem:
         should_delete = (
             TransferChain._should_delete_empty_source_directories(
@@ -111,7 +111,7 @@ def test_mounted_filesystem_detection_is_cached_by_source_directory():
     """
     mounted_filesystem_cache = {}
     with patch(
-            "app.chain._mixins.SystemUtils.is_network_filesystem",
+            "app.chain._transfer.SystemUtils.is_network_filesystem",
             return_value=True,
     ) as is_network_filesystem:
         for _ in range(2):