feat(flutter/admin): requests screen with approve/reject

AdminRequestsScreen renders rows from adminRequestsProvider, joining
requester user_id → username via adminUsersProvider for display
(falls back to UUID prefix when the users list hasn't loaded yet).
Approve fires immediately; Reject confirms via dialog. Both are
optimistic with rollback in the controller.

Adds AdminUsersController build-only (the read side) so the row join
works; mutation methods land in Slice 3 alongside the Users screen.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/admin/admin_providers.dart

@@ -6,6 +6,7 @@ import '../api/endpoints/admin_requests.dart';
 import '../api/endpoints/admin_users.dart';
 import '../library/library_providers.dart' show dioProvider;
 import '../models/admin_request.dart';
+import '../models/admin_user.dart';
 
 final adminRequestsApiProvider = FutureProvider<AdminRequestsApi>((ref) async {
   return AdminRequestsApi(await ref.watch(dioProvider.future));
@@ -89,3 +90,19 @@ class AdminRequestsController extends AsyncNotifier<List<AdminRequest>> {
 final adminRequestsProvider =
     AsyncNotifierProvider<AdminRequestsController, List<AdminRequest>>(
         AdminRequestsController.new);
+
+/// Read side defined here so the Requests screen can join requester
+/// UUID → username for display. Mutation methods (setAdmin,
+/// setAutoApprove, resetPassword, delete) are appended in Slice 3
+/// alongside the Users screen.
+class AdminUsersController extends AsyncNotifier<List<AdminUser>> {
+  @override
+  Future<List<AdminUser>> build() async {
+    final api = await ref.watch(adminUsersApiProvider.future);
+    return api.list();
+  }
+}
+
+final adminUsersProvider =
+    AsyncNotifierProvider<AdminUsersController, List<AdminUser>>(
+        AdminUsersController.new);