Display gallery loading and failure placeholders in the image viewer

tools/chatgui/src/imageviewer_window.cpp

@@ -131,7 +131,8 @@ void ImageViewerWindow::paintEvent(QPaintEvent*) {
 
     if (m_image.isNull()) {
         p.setPen(QColor(150, 150, 150));
-        p.drawText(rect(), Qt::AlignCenter, QCoreApplication::translate("ChatGui", "Failed to load photo"));
+        p.drawText(rect(), Qt::AlignCenter, m_placeholder.isEmpty() ?
+            QCoreApplication::translate("ChatGui", "Failed to load photo") : m_placeholder);
         return;
     }
 

tools/chatgui/src/imageviewer_window.h

@@ -17,6 +17,7 @@ public:
     explicit ImageViewerWindow(QWidget *parent = nullptr);
     void loadFile(const QString& path);
     void loadImage(const QImage& image);
+    void setPlaceholder(const QString& text) { m_placeholder = text; update(); }
 
 signals:
     void closed();
@@ -41,6 +42,7 @@ private:
                 int durMs, double target);
 
     QPixmap m_image;
+    QString m_placeholder;
     double  m_scale = 1.0;
     QPointF m_offset;       /* смещение изображения в пикселях виджета */
     bool    m_dragging = false;