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

ReadMe_EN.md

@@ -390,7 +390,7 @@ Valid     : 2026-02-01 to 2028-02-01
 - **Machine management**: CMachineManager migrated (process / window / network / software / IE history / favorites / service / task / hosts enumeration plus process injection), COMMAND_MACHINE_MANAGE 162
 - **Auth-failed trigger**: new TRIGGER_AUTH_FAILED type (independent of the host-online trigger) pushes plugin DLLs on authorization failure, gated by ShouldLogAuth dedup
 - **iPhone remote audio**: Web Audio API + self-hosted opus-decoder WASM decoding fixes silent audio on iOS without MSE
-- **Quality & transfer controls**: configurable remote-desktop / screen-preview quality-level menus (six presets each) plus a file-transfer speed dialog (seven presets + unlimited, CMD_FILE_SPEED 252)
+- **Quality & transfer controls**: configurable remote-desktop / screen-preview quality-level menus (six presets each) plus a file-transfer speed dialog (seven presets + unlimited, delivered via MasterSettings)
 - **Shine/HELL protocol selection**: choose the protocol when sharing / transferring a host, passed as a trailing colon field, backward compatible
 - **Go web UI subpath deployment**: YAMA_WEB_BASE_PATH mounts the whole web UI under a URL prefix
 - **i18n**: German (de_DE) as the fourth language, a language-switch toolbar button, remaining string translations; remote-desktop clipboard buttons (reachable in fullscreen)