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

common/iniFile.h

@@ -256,6 +256,18 @@ public:
     }
 };
 
+// 文件传输速度：读取 [transfer] 节 ChunkKb / DurationMs，缺省 64KB/50ms（≈1.25MB/s），
+// 并夹到安全范围。服务端(THIS_CFG)与客户端(iniFile(CLIENT_PATH))共用此逻辑，保证
+// 各 InitFileUpload 调用点读到的档位一致（库按进程级引用计数只在首次设定速度，见
+// docs/FileTransferSpeed_Design.md）。
+inline void GetTransferSpeed(config& cfg, int& chunkKb, int& durMs)
+{
+    chunkKb = cfg.GetInt("transfer", "ChunkKb", 64);
+    durMs   = cfg.GetInt("transfer", "DurationMs", 50);
+    if (chunkKb <= 0 || chunkKb > 1024) chunkKb = 64;
+    if (durMs < 0 || durMs > 2000)      durMs = 50;
+}
+
 // 配置读取类: 注册表配置（带键句柄缓存）
 // 注意：缓存操作非线程安全，但竞态条件只会导致少量重复打开，不会崩溃
 class iniFile : public config