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/McpInternal.h

@@ -52,8 +52,20 @@ size_t BoundedStrlen(const char* p, size_t avail);
 std::string ToUtf8(const char* s, UINT cp);
 std::string ToAnsi(const std::string& utf8, UINT cp);
 
+// 严格 UTF-8 → 目标代码页：任一字符无法表示即失败（不用 '?' 顶替）。
+// UTF-8 本身非法也失败。聊天消息用它拒绝 GBK 不可表示的字符（emoji 等）。
+bool Utf8ToCpExact(const std::string& utf8, UINT cp, std::string& out);
+
+// 校验字节串是否为合法 UTF-8（n=0 视为合法）。
+bool IsValidUtf8(const char* s, size_t n);
+
 // 写操作审计：PostMessageA(WM_SHOWERRORMSG) 落到服务端消息/审计日志（不可关闭）。
 void AuditManageAction(CMy2015RemoteDlg* parent, uint64_t devId,
                        const std::string& tool, const std::string& detail);
 
+// 远程交谈审计：与 AuditManageAction 同落地方式，标题区分（MCP远程交谈）。
+// 只记录会话 id / 字节数，绝不记录消息正文。
+void AuditChatAction(CMy2015RemoteDlg* parent, uint64_t devId,
+                     const std::string& tool, const std::string& detail);
+
 } // namespace McpInternal