feat(16-05): wire full sync report + visible sync-request error (D-06, D-21)

- SyncResult interface grows additively: groupMembershipsAdded/Removed
  (D-21 backend gap, existed since Phase 15 but never wired into the
  frontend) plus groupsAdopted/Renamed/Deleted and defaultMarkerMoved
  (Plan 16-03).
- New syncRequestError state: a failed sync request (network error or
  !res.ok) now renders a visible text-sm text-destructive line instead
  of silently reporting a three-zero result as a successful no-op.
- Sync report container gains three new lines: group-membership counts,
  AD-group counts (always visible, even at zero), and a conditional
  amber "default marker reassigned" line shown only when
  defaultMarkerMoved > 0.
- Four new i18n keys under admin.ldap.sync in de.json/en.json.

apps/web/src/app/(portal)/admin/ldap/page.tsx

@@ -63,6 +63,15 @@ interface SyncResult {
   created: number;
   updated: number;
   deactivated: number;
+  // D-21: group-membership reconciliation counters — existed on the backend
+  // since Plan 15/D-21 but were never wired into this interface until now.
+  groupMembershipsAdded: number;
+  groupMembershipsRemoved: number;
+  // Plan 16-03/16-05: group reconciliation counters (SC-3/SC-4/D-05/D-06).
+  groupsAdopted: number;
+  groupsRenamed: number;
+  groupsDeleted: number;
+  defaultMarkerMoved: number;
   errors: string[];
 }
 
@@ -84,6 +93,7 @@ export default function AdminLdapPage() {
   } | null>(null);
   const [syncResult, setSyncResult] = useState<SyncResult | null>(null);
   const [syncing, setSyncing] = useState(false);
+  const [syncRequestError, setSyncRequestError] = useState<string | null>(null);
 
   // Form state for connection settings.
   const [formData, setFormData] = useState({
@@ -265,6 +275,7 @@ export default function AdminLdapPage() {
   const handleSync = async () => {
     setSyncing(true);
     setSyncResult(null);
+    setSyncRequestError(null);
     try {
       const res = await fetch(`${API_URL}/ldap/sync`, {
         method: 'POST',
@@ -274,9 +285,13 @@ export default function AdminLdapPage() {
         const data = await res.json();
         setSyncResult(data);
         await fetchConfig();
+      } else {
+        setSyncResult(null);
+        setSyncRequestError(t('sync.requestError'));
       }
     } catch {
-      setSyncResult({ created: 0, updated: 0, deactivated: 0, errors: ['Network error'] });
+      setSyncResult(null);
+      setSyncRequestError(t('sync.requestError'));
     } finally {
       setSyncing(false);
     }
@@ -1287,6 +1302,12 @@ export default function AdminLdapPage() {
               {syncing ? t('sync.syncing') : t('sync.trigger')}
             </button>
 
+            {/* Sync request failure (Owner-Entscheidung 2026-08-06): a failed
+                sync request must never render as a report full of zeros. */}
+            {syncRequestError && (
+              <p className="text-sm text-destructive">{syncRequestError}</p>
+            )}
+
             {/* Sync result display */}
             {syncResult && (
               <div className="rounded-md border border-border bg-muted/30 p-4">
@@ -1297,6 +1318,26 @@ export default function AdminLdapPage() {
                     deactivated: syncResult.deactivated,
                   })}
                 </p>
+                <p className="text-sm text-muted-foreground">
+                  {t('sync.resultGroupMemberships', {
+                    groupMembershipsAdded: syncResult.groupMembershipsAdded,
+                    groupMembershipsRemoved: syncResult.groupMembershipsRemoved,
+                  })}
+                </p>
+                <p className="text-sm text-muted-foreground">
+                  {t('sync.resultGroups', {
+                    groupsAdopted: syncResult.groupsAdopted,
+                    groupsRenamed: syncResult.groupsRenamed,
+                    groupsDeleted: syncResult.groupsDeleted,
+                  })}
+                </p>
+                {syncResult.defaultMarkerMoved > 0 && (
+                  <p className="text-sm font-medium text-amber-700 dark:text-amber-400">
+                    {t('sync.defaultMarkerMoved', {
+                      defaultMarkerMoved: syncResult.defaultMarkerMoved,
+                    })}
+                  </p>
+                )}
                 {syncResult.errors.length > 0 && (
                   <div className="mt-2 space-y-1">
                     {syncResult.errors.map((err, i) => (