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/pages/License/index.tsx

@@ -25,6 +25,7 @@ interface LicenseStatus {
   payload?: {
     product?: string
     type?: string
+    reason?: string
     limits?: Record<string, number>
     features?: Record<string, boolean>
   }
@@ -130,6 +131,16 @@ export default function LicensePage() {
         />
       )}
 
+      {status?.status !== 'active' && status?.payload?.reason && (
+        <Alert
+          type="error"
+          showIcon
+          message={t('license.serverRejected')}
+          description={status.payload.reason}
+          className="mb-16"
+        />
+      )}
+
       {isTrial && days !== null && days <= 14 && (
         <Alert
           type={days <= 3 ? 'error' : 'warning'}
@@ -226,7 +237,7 @@ export default function LicensePage() {
         <Paragraph type="secondary">{t('license.enterKeyHint')}</Paragraph>
         <Form form={form} layout="vertical" onFinish={(v) => setKey.mutate(v.license_key.trim())}>
           <Form.Item name="license_key" label={t('license.key')} rules={[{ required: true }]}>
-            <Input.TextArea rows={3} placeholder="NMG-XXXX-XXXX-XXXX-XXXX" />
+            <Input.TextArea rows={3} placeholder="NETC-XXXX-XXXX-XXXX-XXXX" />
           </Form.Item>
         </Form>
       </Modal>