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/verification.dart

@@ -0,0 +1,44 @@
+import '../models/verification.dart';
+
+abstract interface class VerificationService {
+  Future<VerificationResult> send({
+    required String email,
+    required String locale,
+  });
+  Future<VerificationResult> confirm({required String token});
+}
+
+/// Explicit preview tokens only. Never use this implementation in production.
+class DummyVerificationService implements VerificationService {
+  const DummyVerificationService({
+    this.delay = const Duration(milliseconds: 600),
+    this.sendFailure,
+  });
+  final Duration delay;
+  final VerificationError? sendFailure;
+
+  @override
+  Future<VerificationResult> send({
+    required String email,
+    required String locale,
+  }) async {
+    await Future<void>.delayed(delay);
+    if (sendFailure case final error?) throw VerificationException(error);
+    return const VerificationResult(isSimulated: true);
+  }
+
+  @override
+  Future<VerificationResult> confirm({required String token}) async {
+    await Future<void>.delayed(delay);
+    switch (token) {
+      case 'demo-valid':
+        return const VerificationResult(isSimulated: true);
+      case 'demo-expired':
+        throw const VerificationException(VerificationError.expired);
+      case 'demo-internal':
+        throw const VerificationException(VerificationError.internal);
+      default:
+        throw const VerificationException(VerificationError.invalid);
+    }
+  }
+}