Feature: Add persistent remote terminal MCP tools

Add terminal_open / terminal_exec / terminal_close so an AI can hold one
shell session per Windows host and run a sequence of commands with cwd and
environment preserved, instead of the one-shot exec_command.

The persistent terminal is a separate full-command write capability gated by
McpTerminal (default off) plus McpReadonly=0, with no whitelist and full
audit. One device maps to one terminal session via the shared m_TermSessions
map; idle sessions are swept after 300s. terminal_exec rejects commands that
contain & or | (they corrupt the sentinel control-operator chain) as well as
control characters.

Also harden exec_command and terminal_exec against newline/CR injection, fix a
dangling subCtx after an abrupt shell disconnect, and refresh lastActiveAt on
command completion. Add en/zh-TW translations for the new UI strings and a
design document covering both exec_command and the persistent terminal.

Co-Authored-By: deepseek-v4-pro

server/2015Remote/McpSettingsDlg.h

@@ -28,12 +28,14 @@ private:
         IDC_MCP_TOKEN  = 1004,  // Token 编辑框
         IDC_MCP_READONLY  = 1005,  // 「只读模式」复选框（默认勾选，禁 exec_command）
         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）
     };
 
     CButton m_btnEnable;
     CStatic m_lblPort, m_lblBind, m_lblToken;
     CEdit   m_editPort, m_editBind, m_editToken;
     CButton m_btnReadonly;
+    CButton m_btnTerminal;
     CStatic m_lblWhitelist;
     CEdit   m_editWhitelist;
     CButton m_btnOK, m_btnCancel;