feat(online): session compte, stockage sécurisé et adapter API (ticket #64)

Ajoute l'adapter API distant (infrastructure/remote/auth_api.dart,
http_api_client.dart) et le stockage sécurisé du token d'auth
(infrastructure/security/secure_storage_auth_token_store.dart), câblés
dans app_bootstrap.dart. Étend le modèle Drift (migration
schemaVersion 9→10) et la couche application (ports, use cases,
entités) pour la session de compte. flutter pub get OK (ajout http,
flutter_secure_storage), build_runner OK, dart format appliqué,
analyze propre (mêmes infos préexistantes), 109/109 tests verts,
build APK debug validé. Premier ticket du chantier "Ajouter les
features serveur au client" (#63).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/application/ports.dart

@@ -15,6 +15,64 @@ abstract interface class IdGenerator {
   String newId();
 }
 
+enum RemoteAuthFailure {
+  invalidCredentials,
+  emailAlreadyUsed,
+  network,
+  unknown,
+}
+
+final class RemoteAuthException implements Exception {
+  const RemoteAuthException(this.failure, [this.message]);
+
+  final RemoteAuthFailure failure;
+  final String? message;
+
+  @override
+  String toString() => message ?? failure.name;
+}
+
+final class RemoteAuthResult {
+  const RemoteAuthResult({
+    required this.userId,
+    required this.email,
+    required this.token,
+    required this.expiresAt,
+  });
+
+  final String userId;
+  final String email;
+  final String token;
+  final DateTime expiresAt;
+}
+
+abstract interface class AuthTokenStore {
+  Future<void> saveToken(String token, DateTime expiresAt);
+  Future<String?> readToken();
+  Future<void> clearToken();
+}
+
+abstract interface class OnlineAccountRepository {
+  Future<UserAccountSession?> currentSession();
+  Future<void> saveSession(UserAccountSession session);
+  Future<void> clearSession();
+}
+
+abstract interface class RemoteAuthApi {
+  Future<RemoteAuthResult> register({
+    required String email,
+    required String password,
+    String? displayName,
+  });
+
+  Future<RemoteAuthResult> login({
+    required String email,
+    required String password,
+  });
+
+  Future<void> logout(String token);
+}
+
 abstract interface class ExerciseRepository {
   Future<Exercise?> findById(String id);
   Future<List<Exercise>> listActive();