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

common/commands.h

@@ -1213,7 +1213,9 @@ typedef struct MasterSettings {
     char        GetPluginUrl[80];           // Since 2026-04-08
     char        Authorized;                 // Since 2026-05-15
     char        IsTrail;                    // Since 2026-05-15
-    char        Reserved[106];              // Since 2025-11-27
+    uint16_t    TransferChunkKb;            // Since 2026-10-07 文件传输速度 chunkKb（随登录配置下发）
+    uint16_t    TransferDurationMs;         // Since 2026-10-07 文件传输速度 durationMs（0=不限速）
+    char        Reserved[102];              // Since 2025-11-27
 } MasterSettings;
 #pragma pack(pop)