Fix: Add missing FileManagerDlg translations to language files

FileManagerDlg.cpp still had several user-facing Chinese strings with no
entry in en_US/zh_TW/de_DE: the "invalid file handle", "no files
selected", "V2 transfer started", "V2 download request sent" and "main
connection not found" messages. Add translations for all five.

The "main connection not found" string contained a literal '=', which
CIniParser treats as the key/value delimiter, so it could never match as
a translation key. Reword it to "clientID %llu" (no '=') so the key
round-trips. The two Mprintf log lines on the V2 checksum path are
log-only and intentionally left untranslated.

Co-Authored-By: deepseek-v4-pro

server/2015Remote/FileManagerDlg.cpp

@@ -3358,7 +3358,7 @@ void CFileManagerDlg::OnTransferV2ToRemote()
             g_2015RemoteDlg->SendFilesToClientV2(mainCtx, files, remoteDir);
             ShowMessage(_TRF("V2传输已启动，共 %d 个文件 -> %s"), (int)files.size(), remoteDir.c_str());
         } else {
-            ShowMessage(_TRF("找不到主连接: clientID=%llu"), clientID);
+            ShowMessage(_TRF("找不到主连接: clientID %llu"), clientID);
         }
     }
 }