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/models/auth.dart

@@ -0,0 +1,56 @@
+/// Ephemeral input: never persist, log, or include passwords in provider state.
+class LoginRequest {
+  const LoginRequest({
+    required this.email,
+    required this.password,
+    required this.locale,
+  });
+  final String email;
+  final String password;
+  final String locale;
+}
+
+class SignupRequest {
+  const SignupRequest({
+    required this.name,
+    required this.email,
+    required this.password,
+    required this.locale,
+  });
+  final String name;
+  final String email;
+  final String password;
+  final String locale;
+}
+
+/// An operation result, not proof of a session or an authorization decision.
+class AuthResult {
+  const AuthResult({required this.userId, required this.isSimulated});
+  final String userId;
+  final bool isSimulated;
+}
+
+enum AuthError { invalidCredentials, emailInUse, unavailable, unexpected }
+
+class AuthException implements Exception {
+  const AuthException(this.code);
+  final AuthError code;
+  @override
+  String toString() => 'AuthException(${code.name})';
+}
+
+enum AuthStatus { idle, loading, success, error }
+
+class AuthState {
+  const AuthState._(this.status, {this.result, this.error});
+  const AuthState.idle() : this._(AuthStatus.idle);
+  const AuthState.loading() : this._(AuthStatus.loading);
+  const AuthState.success(AuthResult result)
+    : this._(AuthStatus.success, result: result);
+  const AuthState.failure(AuthError error)
+    : this._(AuthStatus.error, error: error);
+  final AuthStatus status;
+  final AuthResult? result;
+  final AuthError? error;
+  bool get isLoading => status == AuthStatus.loading;
+}