Feature: Add file-transfer speed dialog with multi-language support

Add a novice-friendly "File Transfer Speed" dialog under the Advanced menu
that controls file-transfer speed in both directions without touching
SimplePlugins. Speed is passed through InitFileUpload's existing
chunkSizeKb/sendDurationMs arguments, so the precompiled library stays
untouched. Seven presets run from ~0.3 MB/s up to a new 10 MB/s tier plus an
unlimited tier; the chosen value is stored in [transfer] ChunkKb/DurationMs.

Upload reads THIS_CFG at the master's InitFileUpload (2015RemoteDlg.cpp:2225)
and takes effect on restart. Download is pushed to the client over a new
CMD_FILE_SPEED (252) command, sent from OnOnlineFileManager and
OnOnlineDesktopManager before each session opens, and read by all four client
InitFileUpload call sites via GetTransferSpeed so the process-global refcount
cannot latch a stale speed. All Chinese strings go through LangManager, with
en_US/zh_TW/de_DE entries added for the tier names, format strings and hint
text.

Co-Authored-By: deepseek-v4-pro

server/2015Remote/2015RemoteDlg.h

@@ -347,6 +347,12 @@ public:
     void SavePreviewProfilesToCfg() const;                // m_PreviewProfiles 落盘到 THIS_CFG
     const PreviewProfile& GetPreviewProfileCfg(int level) const;  // 供 ChooseScreenPreviewParams 用
     void OnPreviewLevels();                               // 高级→屏幕预览质量等级
+
+    // ===== 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）。
+    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);