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/de_DE.ini

@@ -2092,3 +2092,8 @@ TLS
 已将 %d 个 IP 加入白名单=%d IP(s) zur Whitelist hinzugefuegt
 %d 个 IP 已在白名单中=%d IP(s) bereits in der Whitelist
 %d 个 IP 已封禁，无法加入白名单=%d IP(s) sind gesperrt und koennen nicht zur Whitelist hinzugefuegt werden
+%s 文件句柄无效=%s ungueltiges Datei-Handle
+没有选中任何文件=Keine Dateien ausgewaehlt
+V2传输已启动，共 %d 个文件 -> %s=V2-Uebertragung gestartet, %d Datei(en) -> %s
+V2下载请求已发送，共 %d 个项目=V2-Download-Anfrage gesendet, %d Element(e)
+找不到主连接: clientID %llu=Hauptverbindung nicht gefunden: clientID=%llu