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/lang/zh_TW.ini

@@ -2041,3 +2041,20 @@ Minimal
 JPEG质量=JPEG品質
 最大宽度为期望编码宽度；JPEG 质量 1~100=最大寬度為期望編碼寬度；JPEG 品質 1~100
 预览质量等级已保存，下次发送预览时生效。=預覽品質等級已儲存，下次傳送預覽時生效。
+
+; ============================================
+; File Transfer Speed Dialog (文件传输速度)
+; ============================================
+文件传输速度(&F)...=檔案傳輸速度(&F)...
+文件传输速度=檔案傳輸速度
+拖动滑块设置文件传输速度：=拖曳滑桿設定檔案傳輸速度：
+最慢=最慢
+慢=慢
+默认=預設
+快=快
+更快=更快
+很快=很快
+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。