feat(fixtures): add initial admin user fixture

Creates a new data fixture file to seed an initial administrator user
with the email 'admin@example.com'.

This user is loaded into the in-memory repository on startup, providing
a pre-existing privileged account for development and testing of the
dashboard login flow. This is the correct, secure way to provision
initial admin users, removing the need for insecure logic in the public
API services.

Author: fulleni

lib/src/fixtures/user_fixtures.dart

@@ -0,0 +1,18 @@
+import 'package:ht_shared/ht_shared.dart';
+
+/// A list of initial user data to be loaded into the in-memory user repository.
+///
+/// This list includes a pre-configured administrator user, which is essential
+/// for accessing the dashboard in a development environment.
+final List<User> userFixtures = [
+  // The initial administrator user.
+  User(
+    id: 'admin-user-id', // A fixed, predictable ID for the admin.
+    email: 'admin@example.com',
+    roles: const [UserRoles.standardUser, UserRoles.admin],
+    createdAt: DateTime.now().toUtc(),
+  ),
+  // Add other initial users for testing if needed.
+  // Example: A standard user
+  // User( ... ),
+];