Update inbox label localization from English to German in app.cpp to improve user interface consistency and enhance localization efforts.

src/app.cpp

@@ -1011,7 +1011,7 @@ void App::showUnreadMessages(Wt::Json::Object data) {
     Wt::WString buttonText = (int)data["data"] == 0 ? base : Wt::WString("{1} ({2})").arg(base).arg((int)data["data"]);
     auto currentText = inbox_->text();
     inbox_->setText(buttonText);
-    std::string pattern{"Inbox\\(\\s*(\\d+)\\)"};
+    std::string pattern{"Posteingang\\(\\s*(\\d+)\\)"};
     std::regex regex(pattern);
     std::string text = currentText.toUTF8();
     std::smatch match;