feat(15-06): group member management + delete dialog with concrete impact numbers

- GroupMembersModal.tsx: chip list of current members with source badge
  (MANUAL/LDAP); LDAP-sourced chips carry a disabled remove button with a
  "managed via AD sync" tooltip (D-19) instead of an active one; second
  section adds manual members via GET /users + POST /groups/:id/members,
  already-member candidates shown disabled (upsert on the API is
  folgenlos, no special-case needed)
- DeleteGroupDialog.tsx: loads GET /groups/:id/impact and interpolates
  memberCount/grantCount into the confirmation text (D-17); deliberately
  breaks from the project's silent-delete-failure precedent -- stays open
  and shows a visible error on a failed DELETE, since a silent failure
  here would leave an admin believing a group (and its grants) is gone
  while it still grants access (T-15-24)
- page.tsx: wires both dialogs in, refetches the group list after any
  member/delete mutation so member counts and badges stay current
- groups-page.test.tsx: disabled-vs-active remove button by membership
  source, delete text shows both numbers, visible error + dialog stays
  open on failed delete

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

@@ -4,6 +4,8 @@ import { useCallback, useEffect, useState } from 'react';
 import { useTranslations } from 'next-intl';
 import { useAuthStore } from '@/lib/stores/auth-store';
 import { GroupFormModal } from './components/GroupFormModal';
+import { GroupMembersModal } from './components/GroupMembersModal';
+import { DeleteGroupDialog } from './components/DeleteGroupDialog';
 
 const API_URL = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:3001';
 
@@ -252,8 +254,21 @@ export default function AdminGroupsPage() {
         />
       )}
 
-      {/* GroupMembersModal / DeleteGroupDialog are wired in here by Task 3
-          (member/delete state above is already in place for that). */}
+      {membersGroup && (
+        <GroupMembersModal
+          group={membersGroup}
+          onClose={() => setMembersGroup(null)}
+          onChanged={fetchGroups}
+        />
+      )}
+
+      {deleteTarget && (
+        <DeleteGroupDialog
+          group={deleteTarget}
+          onClose={() => setDeleteTarget(null)}
+          onDeleted={fetchGroups}
+        />
+      )}
     </div>
   );
 }