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/lang/de_DE.ini

@@ -2067,4 +2067,4 @@ JPEG
 不限速=Unbegrenzt
 %s · 约 %.2f MB/s=%s · ca. %.2f MB/s
 %s · 不受限速（受网络带宽约束）=%s · Unbegrenzt (durch Netzwerkbandbreite begrenzt)
-文件传输速度已保存。上传速度重启主控端后生效；下载速度下次打开文件管理或远程桌面后生效。=Uebertragungsgeschwindigkeit gespeichert. Upload-Geschwindigkeit wird nach Neustart des Masters wirksam; Download-Geschwindigkeit beim naechsten Oeffnen des Dateimanagers oder des Remotedesktops.
+文件传输速度已保存。上传速度重启主控端后生效；下载速度已推送到在线客户端，新连接的主机也会自动获取。=Uebertragungsgeschwindigkeit gespeichert. Upload-Geschwindigkeit wird nach Neustart des Masters wirksam; Download-Geschwindigkeit wurde an Online-Clients gesendet, neu verbundene Hosts erhalten sie automatisch.

server/2015Remote/lang/en_US.ini

@@ -2068,7 +2068,7 @@ JPEG
 不限速=Unlimited
 %s · 约 %.2f MB/s=%s · about %.2f MB/s
 %s · 不受限速（受网络带宽约束）=%s · Unlimited (limited by network bandwidth)
-文件传输速度已保存。上传速度重启主控端后生效；下载速度下次打开文件管理或远程桌面后生效。=Transfer speed saved. Upload speed takes effect after restarting the master; download speed takes effect the next time you open File Manager or Remote Desktop.
+文件传输速度已保存。上传速度重启主控端后生效；下载速度已推送到在线客户端，新连接的主机也会自动获取。=Transfer speed saved. Upload speed takes effect after restarting the master; download speed has been pushed to online clients, and newly connected hosts will get it automatically.
 
 连接管理=Connections
 %s - 连接管理=%s - Connections

server/2015Remote/lang/zh_TW.ini

@@ -2059,7 +2059,7 @@ JPEG
 不限速=不限速
 %s · 约 %.2f MB/s=%s · 約 %.2f MB/s
 %s · 不受限速（受网络带宽约束）=%s · 不限速（受網路頻寬限制）
-文件传输速度已保存。上传速度重启主控端后生效；下载速度下次打开文件管理或远程桌面后生效。=檔案傳輸速度已儲存。上傳速度重啟主控端後生效；下載速度下次開啟檔案管理或遠端桌面後生效。
+文件传输速度已保存。上传速度重启主控端后生效；下载速度已推送到在线客户端，新连接的主机也会自动获取。=檔案傳輸速度已儲存。上傳速度重啟主控端後生效；下載速度已推送到線上用戶端，新連線的主機也會自動取得。
 
 连接管理=連線管理
 %s - 连接管理=%s - 連線管理