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/sxchnge_auth.dart

@@ -0,0 +1,12 @@
+/// Platform-independent authentication state and injectable services.
+library;
+
+export 'src/models/auth.dart';
+export 'src/providers/auth.dart';
+export 'src/providers/login.dart';
+export 'src/providers/signup.dart';
+export 'src/services/contracts.dart';
+export 'src/services/dummy.dart';
+export 'src/models/verification.dart';
+export 'src/providers/verification.dart';
+export 'src/services/verification.dart';