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/providers/auth.dart

@@ -0,0 +1,46 @@
+import 'package:flutter/foundation.dart';
+
+import '../models/auth.dart';
+
+/// Shared operation lifecycle adapted from Sendico's ChangeNotifier providers.
+/// Reset invalidates pending results; it does not cancel service-side work.
+abstract class AuthProvider extends ChangeNotifier {
+  AuthState _state = const AuthState.idle();
+  int _generation = 0;
+  bool _disposed = false;
+  AuthState get state => _state;
+
+  @protected
+  Future<void> run(Future<AuthResult> Function() operation) async {
+    if (_disposed || _state.isLoading) return;
+    final generation = ++_generation;
+    _state = const AuthState.loading();
+    notifyListeners();
+    try {
+      final result = await operation();
+      if (_disposed || generation != _generation) return;
+      _state = AuthState.success(result);
+    } catch (error) {
+      if (_disposed || generation != _generation) return;
+      // Never retain or expose arbitrary exceptions that may contain credentials.
+      _state = AuthState.failure(
+        error is AuthException ? error.code : AuthError.unexpected,
+      );
+    }
+    if (!_disposed && generation == _generation) notifyListeners();
+  }
+
+  void reset() {
+    if (_disposed) return;
+    ++_generation;
+    _state = const AuthState.idle();
+    notifyListeners();
+  }
+
+  @override
+  void dispose() {
+    _disposed = true;
+    ++_generation;
+    super.dispose();
+  }
+}