Fix: Deliver file-transfer speed via MasterSettings (was CMD_FILE_SPEED)

The file-transfer speed dialog previously sent a separate CMD_FILE_SPEED
(5 bytes) back-to-back before COMMAND_SCREEN_SPY / COMMAND_LIST_DRIVE when
opening remote desktop or file manager. The IOCP send path batches two
consecutive Send2Client calls into one WSASend of the accumulated
OutCompressedBuffer, so the client received the [FILE_SPEED][SCREEN_SPY]
command stream twice - the root cause of two remote-desktop dialogs popping
up on a single click (introduced in commit 76decec).

Fold the two values into MasterSettings (TransferChunkKb / TransferDurationMs,
uint16 each, before Reserved, sizeof unchanged at 1000) so they reach every
client on login and are broadcast to online clients when the dialog saves.
The client persists them into [transfer] on CMD_MASTERSETTING, guarded by
TransferChunkKb != 0 so older masters keep local defaults. CMD_FILE_SPEED(252)
and its client handler remain for backward compatibility, but the master no
longer sends it.

Update the design doc, the three GBK lang files' prompt entry, and the READMEs.

Co-Authored-By: deepseek-v4-pro

server/2015Remote/2015RemoteDlg.h

@@ -351,9 +351,8 @@ public:
 
     // ===== 文件传输速度配置（[transfer] 节，ChunkKb / DurationMs） =====
     // 主控端 UI 偏好，持久化走 THIS_CFG。上传方向读 THIS_CFG（重启主控端生效）；
-    // 下载方向经 CMD_FILE_SPEED 下发到被控端（下次打开文件管理/远程桌面会话生效）。
+    // 下载方向随 MasterSettings 下发（新连接登录即收到，改档后立即推送给在线客户端）。
     void OnFileTransferSpeed();          // 高级→文件传输速度
-    void SendFileSpeedCommand();         // 把当前档位经主连接下发给被控端
     void SendThumbnailRequest(class context* ctx);// 发一次缩略图请求
     void CacheThumbnail(uint64_t clientID, const BYTE* jpeg, size_t bytes);
     void ClearThumbnailCacheEntry(uint64_t clientID);