Add admin user management API and frontend UI

Backend:
- /api/admin/users: GET list, POST create, PUT update, DELETE
- Admin users can be added with email, display name, and password
- Self-deletion and last-admin deletion are blocked
- Admin role auto-assigned on creation

Frontend:
- AdminUsersTabComponent: list + add/delete admin users
- AdminUserService: HTTP client for admin user CRUD
- AdminUser interface
- 'Admins' tab added to admin dashboard

Tests:
- 10 new tests for admin user management endpoints
- 140 total tests pass (130 original + 10 new)

src/app/admin/admin.component.html

@@ -17,6 +17,7 @@
       <button class="tab-btn" [class.active]="activeTab() === 'underwriters'" (click)="activeTab.set('underwriters')">Underwriters</button>
       <button class="tab-btn" [class.active]="activeTab() === 'mobile-builds'" (click)="activeTab.set('mobile-builds')">Mobile Builds</button>
       <button class="tab-btn" [class.active]="activeTab() === 'stats'" (click)="activeTab.set('stats')">Stats</button>
+      <button class="tab-btn" [class.active]="activeTab() === 'admins'" (click)="activeTab.set('admins')">Admins</button>
     </div>
 
     <!-- Tab content -->
@@ -73,6 +74,10 @@
       <app-admin-stats-dashboard />
     </div>
     }
+
+    @if (activeTab() === 'admins') {
+    <app-admin-users-tab />
+    }
   </div>
 
   <!-- Form modals -->