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/contracts.dart

@@ -0,0 +1,11 @@
+import '../models/auth.dart';
+
+/// Production adapters own transport, DTO mapping, and authentication effects.
+/// Providers and widgets only consume domain results and typed failures.
+abstract interface class LoginService {
+  Future<AuthResult> login(LoginRequest request);
+}
+
+abstract interface class SignupService {
+  Future<AuthResult> signup(SignupRequest request);
+}