Copertine: validazione contenuto prima del caching (niente più HTML in cache), id fisso per il dettaglio, handler con sintassi moderna

qml/pages/BookDetailPage.qml

@@ -172,13 +172,14 @@ Page {
 
     Component.onCompleted: {
         if (bookData.coverUrl && bookData.coverUrl.length > 0)
-            apiClient.fetchImage(bookData.coverUrl, "detail:" + bookData.id)
+            apiClient.fetchImage(bookData.coverUrl, "detailcover")
     }
 
     Connections {
         target: apiClient
-        onImageReady: {
-            if (id === "detail:" + bookData.id)
+        // nota: parametro rinominato (imageId) — "id" è parola chiave QML e rompe il confronto
+        onImageReady: function(imageId, localPath) {
+            if (imageId === "detailcover")
                 page.coverSource = localPath
         }
     }

qml/pages/FeedPage.qml

@@ -175,9 +175,9 @@ Page {
             page.loadingMore = false
             page.errorMessage = message
         }
-        onImageReady: {
+        onImageReady: function(imageId, localPath) {
             for (var i = 0; i < feedModel.count; i++) {
-                if (feedModel.get(i).coverId === id) {
+                if (feedModel.get(i).coverId === imageId) {
                     feedModel.setProperty(i, "coverSource", localPath)
                     break
                 }

src/apiclient.cpp

@@ -127,6 +127,13 @@ void ApiClient::getFeed(const QString &url)
     });
 }
 
+static bool looksLikeImage(const QByteArray &data)
+{
+    // sniff dei magic byte per PNG/JPEG/GIF/WebP senza dipendere da QtGui
+    return data.startsWith("\x89PNG") || data.startsWith("\xFF\xD8")
+            || data.startsWith("GIF8") || data.startsWith("RIFF");
+}
+
 void ApiClient::fetchImage(const QString &url, const QString &id)
 {
     if (url.isEmpty()) {
@@ -150,16 +157,27 @@ void ApiClient::fetchImage(const QString &url, const QString &id)
     }
 
     QNetworkReply *reply = startGet(imageUrl);
-    connect(reply, &QNetworkReply::finished, this, [this, reply, id, localPath]() {
+    connect(reply, &QNetworkReply::finished, this, [this, reply, id, localPath, imageUrl]() {
         reply->deleteLater();
         const int status = reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();
         if (reply->error() != QNetworkReply::NoError || status >= 400) {
             emit imageReady(id, QString());
             return;
         }
+        const QByteArray body = reply->readAll();
+        const QByteArray contentType = reply->header(
+                QNetworkRequest::ContentTypeHeader).toByteArray();
+        // non cachare risposte che non sono immagini (es. pagine HTML di errore 200)
+        const bool isImage = contentType.startsWith("image/")
+                || (contentType.isEmpty() && looksLikeImage(body));
+        if (!isImage) {
+            qDebug() << "fetchImage: risposta non immagine scartata" << imageUrl;
+            emit imageReady(id, QString());
+            return;
+        }
         QFile file(localPath);
         if (file.open(QIODevice::WriteOnly)) {
-            file.write(reply->readAll());
+            file.write(body);
             file.close();
             emit imageReady(id, localPath);
         } else {