[UI] Corrected regression on defaut cover

src/ui/resource/CoverResource.cpp

@@ -87,7 +87,7 @@ CoverResource::getTrackUrl(Database::Track::id_type trackId, std::size_t size) c
 std::string
 CoverResource::getUnkownTrackUrl(size_t size) const
 {
-	return Wt::WApplication::instance()->docRoot() + unknownCoverPath + "&size=" + std::to_string(size);
+	return url() + "&size=" + std::to_string(size);
 }
 
 void