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/verification.dart

@@ -0,0 +1,73 @@
+import 'package:flutter/foundation.dart';
+
+import '../models/verification.dart';
+import '../services/verification.dart';
+
+class VerificationProvider extends ChangeNotifier {
+  VerificationProvider(this.service);
+  final VerificationService service;
+  VerificationState _state = const VerificationState(VerificationStatus.idle);
+  VerificationState get state => _state;
+  bool _disposed = false;
+  int _generation = 0;
+
+  Future<void> send({required String email, required String locale}) => _run(
+    () => service.send(email: email.trim(), locale: locale),
+    VerificationStatus.sending,
+    VerificationStatus.sent,
+  );
+  Future<void> confirm(String token) => _run(
+    () => token.trim().isEmpty
+        ? Future.error(const VerificationException(VerificationError.invalid))
+        : service.confirm(token: token),
+    VerificationStatus.checking,
+    VerificationStatus.verified,
+  );
+
+  Future<void> _run(
+    Future<VerificationResult> Function() operation,
+    VerificationStatus loading,
+    VerificationStatus success,
+  ) async {
+    if (_disposed || _state.isLoading) return;
+    final generation = ++_generation;
+    _state = VerificationState(loading);
+    notifyListeners();
+    if (_disposed || generation != _generation) return;
+    try {
+      final result = await operation();
+      if (_disposed || generation != _generation) return;
+      _state = VerificationState(success, isSimulated: result.isSimulated);
+    } catch (error) {
+      if (_disposed || generation != _generation) return;
+      final code = error is VerificationException
+          ? error.code
+          : VerificationError.internal;
+      // Send failures must not be presented as confirmation-token failures.
+      _state = VerificationState(
+        loading == VerificationStatus.sending
+            ? VerificationStatus.error
+            : switch (code) {
+                VerificationError.expired => VerificationStatus.expired,
+                VerificationError.invalid => VerificationStatus.invalid,
+                VerificationError.internal => VerificationStatus.error,
+              },
+      );
+    }
+    notifyListeners();
+  }
+
+  void reset() {
+    if (_disposed) return;
+    ++_generation;
+    _state = const VerificationState(VerificationStatus.idle);
+    notifyListeners();
+  }
+
+  @override
+  void dispose() {
+    _disposed = true;
+    ++_generation;
+    super.dispose();
+  }
+}