fix(thumbnail): it was commented out by mistake (#3403)

modes/longitudinal/src/index.js

@@ -194,7 +194,7 @@ function modeFactory() {
           return {
             id: ohif.layout,
             props: {
-              // leftPanels: [tracked.thumbnailList],
+              leftPanels: [tracked.thumbnailList],
               rightPanels: [dicomSeg.panel, tracked.measurements],
               rightPanelDefaultClosed: true,
               viewports: [