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/lang/en_US.ini

@@ -2107,3 +2107,8 @@ TLS
 已将 %d 个 IP 加入白名单=Added %d IP(s) to the whitelist
 %d 个 IP 已在白名单中=%d IP(s) already in the whitelist
 %d 个 IP 已封禁，无法加入白名单=%d IP(s) are banned and cannot be added to the whitelist
+%s 文件句柄无效=%s invalid file handle
+没有选中任何文件=No files selected
+V2传输已启动，共 %d 个文件 -> %s=V2 transfer started, %d file(s) -> %s
+V2下载请求已发送，共 %d 个项目=V2 download request sent, %d item(s)
+找不到主连接: clientID %llu=Main connection not found: clientID=%llu