Feature: Add MCP tools for remote chat with the client user

Add chat_open, chat_send, chat_wait and chat_close to the MCP tool set,
behind a new McpChat switch that defaults to off and is independent of
the read-only switch. The tools reuse the existing client chat link: the
main connection sends COMMAND_CHAT, the client opens a sub-connection
with TOKEN_CONN_AUTH and TOKEN_CHAT_START, the server answers with
COMMAND_NEXT_CHAT, and both directions then carry raw CP936 text.

Sessions are per host and held in shared pointers under their own mutex,
so a waiting chat_wait survives a close or a dropped sub-connection, and
chat_send is serialised per session. Text is converted strictly between
UTF-8 and CP936: invalid UTF-8, characters GBK cannot represent (emoji,
accented letters) and messages over the client's 4000-byte buffer are
rejected with -32602 instead of being mangled. A one-second settle delay
after the sub-connection binds keeps the first message from reaching the
client before its chat window exists.

The routing hooks run before the host-management hooks in MessageHandle
and clear the route in OfflineProc before the context leaves the list. A
five-second tombstone absorbs a late TOKEN_CHAT_START after a timeout, so
a failed chat_open does not pop the MFC chat dialog. Idle sessions are
swept on every tool call, and audit entries record the session id and
byte counts only, never message bodies.

The settings dialog gains the chat checkbox and the translated labels
for all three languages, and the strict UTF-8 to CP936 helpers move from
McpMachine.cpp to McpInternal so both tool families can share them.

Co-Authored-By: deepseek-flash

server/2015Remote/McpSettingsDlg.h

@@ -34,6 +34,7 @@ private:
         IDC_MCP_MANAGE      = 1010,  // 「启用管理操作」复选框（kill/窗口/服务/电源，要求只读关）
         IDC_MCP_MACHINE     = 1011,  // 「启用主机管理」复选框（host_* 工具族）
         IDC_MCP_MACHINE_DANGEROUS = 1012,  // 「启用危险主机操作」复选框（冻结/停止/重启）
+        IDC_MCP_CHAT        = 1013,  // 「启用远程交谈」复选框（chat_* 工具族，不要求只读关）
     };
 
     CButton m_btnEnable;
@@ -46,6 +47,7 @@ private:
     CButton m_btnManage;
     CButton m_btnMachine;
     CButton m_btnMachineDangerous;
+    CButton m_btnChat;
     CStatic m_lblWhitelist;
     CEdit   m_editWhitelist;
     CButton m_btnOK, m_btnCancel;