fix(license): Rejection-Reason anzeigen + Placeholder NETC- korrigieren

Wenn der License-Server valid:false zurückgibt, wird payload.reason
jetzt als roter Alert angezeigt — bisher war der Grund nicht sichtbar
und der Operator sah nur "Abgelaufen" ohne Erklärung.

Formular-Placeholder NMG-XXXX → NETC-XXXX (mail-gateway copy-paste).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

management-ui/src/i18n/locales/en/common.json

@@ -1071,6 +1071,7 @@
     "confirmClear": "Really remove the license key?",
     "confirmClearHint": "The system will fall back to trial-mode once the key is deleted.",
     "lastVerifyFailed": "Last server verify failed",
+    "serverRejected": "License rejected by server",
     "trialExpiring": "Trial expires in {{days}} day(s)",
     "trialExpiringHint": "Activate a license before the trial period ends.",
     "tagActive": "Active",