Add shared authentication and email verification services

Provide reusable models, injectable service contracts, lifecycle-safe providers and explicit dummy implementations for login, signup and verification. Add focused error and async lifecycle tests. Related to #4 and #5.

frontend/packages/sxchnge_auth/lib/src/services/dummy.dart

@@ -0,0 +1,36 @@
+import '../models/auth.dart';
+import 'contracts.dart';
+
+/// Deterministic local simulation. No network, storage, credentials or sessions
+/// are retained. Configure failures explicitly rather than using magic emails.
+class DummyLoginService implements LoginService {
+  const DummyLoginService({
+    this.delay = const Duration(milliseconds: 600),
+    this.failure,
+  });
+  final Duration delay;
+  final AuthError? failure;
+
+  @override
+  Future<AuthResult> login(LoginRequest request) async {
+    await Future<void>.delayed(delay);
+    if (failure case final error?) throw AuthException(error);
+    return const AuthResult(userId: 'demo-user', isSimulated: true);
+  }
+}
+
+class DummySignupService implements SignupService {
+  const DummySignupService({
+    this.delay = const Duration(milliseconds: 600),
+    this.failure,
+  });
+  final Duration delay;
+  final AuthError? failure;
+
+  @override
+  Future<AuthResult> signup(SignupRequest request) async {
+    await Future<void>.delayed(delay);
+    if (failure case final error?) throw AuthException(error);
+    return const AuthResult(userId: 'demo-user', isSimulated: true);
+  }
+}