fix: use runtime detection for Wayland clipboard (flameshot-org#4497)

When compiled with USE_WAYLAND_CLIPBOARD, the clipboard now works
correctly on both Wayland and X11 sessions.

Previously, builds with USE_WAYLAND_CLIPBOARD=ON would always use
KSystemClipboard, which doesn't work properly when running under X11
(e.g., via XWayland or QT_QPA_PLATFORM=xcb).

This change adds a runtime check for the platform name:
- 'wayland' -> uses KSystemClipboard (KF6 GuiAddons)
- otherwise -> uses QApplication::clipboard() (standard Qt)

This allows distros to ship a single binary that works on both
display servers without requiring separate builds.

Co-authored-by: Benigno Batista Júnior <benigno@opus-software.com.br>

Author: benignobjunior

src/utils/screenshotsaver.cpp

@@ -171,11 +171,16 @@ void saveToClipboardMime(const QPixmap& capture, const QString& imageType)
         auto* mimeData = new QMimeData();
 
 #ifdef USE_WAYLAND_CLIPBOARD
-        mimeData->setImageData(formattedPixmap.toImage());
-        mimeData->setData(QStringLiteral("x-kde-force-image-copy"),
-                          QByteArray());
-        KSystemClipboard::instance()->setMimeData(mimeData,
-                                                  QClipboard::Clipboard);
+        if (QGuiApplication::platformName() == "wayland") {
+            mimeData->setImageData(formattedPixmap.toImage());
+            mimeData->setData(QStringLiteral("x-kde-force-image-copy"),
+                              QByteArray());
+            KSystemClipboard::instance()->setMimeData(mimeData,
+                                                      QClipboard::Clipboard);
+        } else {
+            mimeData->setData("image/" + imageType, array);
+            QApplication::clipboard()->setMimeData(mimeData);
+        }
 #else
         mimeData->setData("image/" + imageType, array);
         QApplication::clipboard()->setMimeData(mimeData);