fix(sessions): lista vuota sul device — modello C++ con ruoli (v0.1.2)

Causa: su Qt 5.6 (kit Sailfish 5.1) una QVariantList di QVariantMap
usata come model non fa risolvere i campi nei delegate QML
(model.title/message_count/updated_at = undefined -> righe vuote), pur
con payload corretto dal server (verificato con cattura reale del 12/09).

- Nuovo src/sessionsmodel.{h,cpp}: QAbstractListModel con ruoli
  session_id/title/message_count/updated_at; api.sessions ora e' il
  modello, api.sessionsCount per lo stato "nessuna sessione"
- /api/sessions con sidebar_source=webui&exclude_hidden=1 (come la
  sidebar del browser: solo sessioni WebUI, niente CLI/cron/nascoste)
- SessionsPage: describe(message_count, updated_at) con parametri espliciti
- PROTOCOL.md: payload sessions verificato + nota del pitfall Qt 5.6

src/apiclient.cpp

@@ -15,6 +15,7 @@ ApiClient::ApiClient(Settings *settings, QObject *parent)
     : QObject(parent)
     , m_settings(settings)
     , m_jar(new CookieJar(this))
+    , m_sessionsModel(new SessionsModel(this))
     , m_loggedIn(false)
     , m_busy(false)
     , m_streaming(false)
@@ -170,7 +171,7 @@ void ApiClient::logout()
     });
     m_jar->clear();
     setLoggedIn(false);
-    m_sessions.clear();
+    m_sessionsModel->setSessions(QVariantList());
     emit sessionsChanged();
     m_session.clear();
     emit sessionChanged();
@@ -181,7 +182,14 @@ void ApiClient::logout()
 void ApiClient::refreshSessions()
 {
     setBusy(true);
-    QNetworkReply *reply = m_nam.get(jsonRequest(QStringLiteral("/api/sessions")));
+    // Stessi parametri della sidebar del WebUI: solo sessioni WebUI visibili
+    // (senza parametri arriverebbero anche le CLI/cron e le nascoste).
+    QUrl url = apiUrl(QStringLiteral("/api/sessions"));
+    QUrlQuery q;
+    q.addQueryItem(QStringLiteral("sidebar_source"), QStringLiteral("webui"));
+    q.addQueryItem(QStringLiteral("exclude_hidden"), QStringLiteral("1"));
+    url.setQuery(q);
+    QNetworkReply *reply = m_nam.get(QNetworkRequest(url));
     connect(reply, &QNetworkReply::finished, this, [this, reply]() {
         reply->deleteLater();
         setBusy(false);
@@ -192,11 +200,16 @@ void ApiClient::refreshSessions()
         }
         const QJsonObject obj = QJsonDocument::fromJson(body).object();
         const QJsonArray arr = obj.value(QStringLiteral("sessions")).toArray();
-        m_sessions = arr.toVariantList();
+        m_sessionsModel->setSessions(arr.toVariantList());
         emit sessionsChanged();
     });
 }
 
+int ApiClient::sessionsCount() const
+{
+    return m_sessionsModel->rowCount();
+}
+
 void ApiClient::openSession(const QString &id)
 {
     setLastError(QString());