Remove unused variable warnings

Commented out unused variable names in function parameters
Casted unused variable (void) in for-loop
Fixed a signed-unsigned comparison warning

src/contentmanager.cpp

@@ -63,6 +63,7 @@ QStringList ContentManager::getBookInfos(QString id, const QStringList &keys)
 
     if (nullptr == b){
         for(auto& key:keys) {
+            (void) key;
             values.append("");
         }
         return values;
@@ -161,8 +162,10 @@ QStringList ContentManager::updateDownloadInfos(QString id, const QStringList &k
 {
     QStringList values;
     if (!mp_downloader) {
-        for(auto& key: keys)
+        for(auto& key: keys) {
+            (void) key;
             values.append("");
+        }
         return values;
     }
     auto& b = mp_library->getBookById(id);
@@ -255,7 +258,8 @@ QString ContentManager::downloadBook(const QString &id)
     }();
     auto downloadPath = KiwixApp::instance()->getSettingsManager()->getDownloadDir();
     QStorageInfo storage(downloadPath);
-    if (book.getSize() > storage.bytesAvailable()) {
+    auto bytesAvailable = storage.bytesAvailable();
+    if (bytesAvailable == -1 || book.getSize() > (unsigned) bytesAvailable) {
         return "storage_error";
     }
     auto booksList = mp_library->getBookIds();

src/webpage.cpp

@@ -14,7 +14,7 @@ WebPage::WebPage(QObject *parent) :
     action(QWebEnginePage::Reload)->setVisible(false);
 }
 
-bool WebPage::acceptNavigationRequest(const QUrl &url, QWebEnginePage::NavigationType type, bool isMainFrame)
+bool WebPage::acceptNavigationRequest(const QUrl &url, QWebEnginePage::NavigationType /*type*/, bool /*isMainFrame*/)
 {
     if (url.scheme() != "zim") {
         QDesktopServices::openUrl(url);