Feature: Add download_file MCP tool (V2 protocol, SHA-256 verified)

download_file pulls a remote file or directory back to the controller over
the existing V2 file-transfer protocol. It reuses COMMAND_LIST_DRIVE to open
the file-manager sub-link, then sends CMD_DOWN_FILES_V2 so the Windows client
streams COMMAND_SEND_FILE_V2 chunks and a per-file COMMAND_FILE_COMPLETE_V2
SHA-256 checksum over its own authenticated sub-connection. The server routes
those two packet types into a new per-device FileTransferSession whenever a
download is pending, so the headless path never opens the GUI progress dialog;
the C2C and existing GUI branches are left untouched.

Files are written under a normalized local_dir, and every chunk filename is
resolved and verified to stay inside local_dir (directories included) to block
.. traversal; overwrite=false skips existing files and counts them as skipped.
One transfer per host (mutually exclusive with the one-shot pending registry),
a dedicated McpFileTransfer=0-by-default gate surfaced in the settings dialog,
and audit logging complete the change. upload_file remains future work.

Co-Authored-By: deepseek-v4-pro

server/2015Remote/McpSettingsDlg.h

@@ -30,6 +30,7 @@ private:
         IDC_MCP_WHITELIST = 1006,  // 命令白名单编辑框（多行，逗号/换行分隔，空 = 内置只读前缀）
         IDC_MCP_TERMINAL  = 1007,  // 「启用持久终端」复选框（全命令，无白名单，要求只读关）
         IDC_MCP_REMOTECONTROL = 1008,  // 「启用远程控制」复选框（AI 操控桌面，要求只读关）
+        IDC_MCP_FILETRANSFER = 1009,  // 「启用文件传输」复选框（download_file，不要求只读关）
     };
 
     CButton m_btnEnable;
@@ -38,6 +39,7 @@ private:
     CButton m_btnReadonly;
     CButton m_btnTerminal;
     CButton m_btnRemoteControl;
+    CButton m_btnFileTransfer;
     CStatic m_lblWhitelist;
     CEdit   m_editWhitelist;
     CButton m_btnOK, m_btnCancel;