fix: resolve translation interpolation issue for download button

Fixed the download selected button not displaying count properly.
The translation key 'gallery.downloadSelected' was not receiving
the count parameter for interpolation, causing "{{count}}" to
display literally instead of the actual number.

Fixes the issue where the button showed:
- "Download {{count}} Selected" instead of "Download 2 Selected"
- "{{count}} ausgewählte herunterladen" instead of "3 ausgewählte herunterladen"

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/gallery/GallerySidebar.tsx

@@ -194,7 +194,7 @@ export const GallerySidebar: React.FC<GallerySidebarProps> = ({
                     disabled={isDownloading}
                     className="w-full"
                   >
-                    {t('gallery.downloadSelected')} ({selectedCount})
+                    {t('gallery.downloadSelected', { count: selectedCount })} ({selectedCount})
                   </Button>
                 )}
               </div>