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/test/verification_test.dart

@@ -0,0 +1,101 @@
+import 'dart:async';
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:sxchnge_auth/sxchnge_auth.dart';
+
+class PendingService implements VerificationService {
+  final pending = Completer<VerificationResult>();
+  int calls = 0;
+  String? email;
+  String? locale;
+  @override
+  Future<VerificationResult> send({
+    required String email,
+    required String locale,
+  }) {
+    calls++;
+    this.email = email;
+    this.locale = locale;
+    return pending.future;
+  }
+
+  @override
+  Future<VerificationResult> confirm({required String token}) {
+    calls++;
+    return pending.future;
+  }
+}
+
+void main() {
+  for (final scenario in {
+    'demo-valid': VerificationStatus.verified,
+    'demo-expired': VerificationStatus.expired,
+    'bad': VerificationStatus.invalid,
+    '': VerificationStatus.invalid,
+    'demo-internal': VerificationStatus.error,
+  }.entries) {
+    test('confirmation maps ${scenario.key} to ${scenario.value}', () async {
+      final provider = VerificationProvider(
+        const DummyVerificationService(delay: Duration.zero),
+      );
+      addTearDown(provider.dispose);
+      await provider.confirm(scenario.key);
+      expect(provider.state.status, scenario.value);
+      expect(
+        provider.state.isSimulated,
+        scenario.value == VerificationStatus.verified,
+      );
+    });
+  }
+  test(
+    'send trims email, forwards locale and suppresses duplicate requests',
+    () async {
+      final service = PendingService();
+      final provider = VerificationProvider(service);
+      addTearDown(provider.dispose);
+      final first = provider.send(email: ' alex@example.com ', locale: 'ru');
+      expect(provider.state.status, VerificationStatus.sending);
+      await provider.send(email: 'other@example.com', locale: 'en');
+      expect(service.calls, 1);
+      expect(service.email, 'alex@example.com');
+      expect(service.locale, 'ru');
+      service.pending.complete(const VerificationResult(isSimulated: true));
+      await first;
+      expect(provider.state.status, VerificationStatus.sent);
+      expect(provider.state.isSimulated, isTrue);
+    },
+  );
+  test('reset discards late result', () async {
+    final service = PendingService();
+    final provider = VerificationProvider(service);
+    addTearDown(provider.dispose);
+    final operation = provider.confirm('token');
+    provider.reset();
+    service.pending.complete(const VerificationResult(isSimulated: true));
+    await operation;
+    expect(provider.state.status, VerificationStatus.idle);
+  });
+  test('dispose discards late exception', () async {
+    final service = PendingService();
+    final provider = VerificationProvider(service);
+    final operation = provider.confirm('token');
+    provider.dispose();
+    service.pending.completeError(StateError('private error'));
+    await operation;
+  });
+  test(
+    'send errors are internal errors even for token exception types',
+    () async {
+      final provider = VerificationProvider(
+        const DummyVerificationService(
+          delay: Duration.zero,
+          sendFailure: VerificationError.expired,
+        ),
+      );
+      addTearDown(provider.dispose);
+      await provider.send(email: 'alex@example.com', locale: 'en');
+      expect(provider.state.status, VerificationStatus.error);
+    },
+  );
+}