Update grid layout, refetch interval, and image handling

Adjusted the grid column behavior for medium screens in event themes and commented out unused badge rendering. Updated the image uploader to handle `existingImage` using the server URL and added debugging output. Modified the event theme context to use a fixed refetch interval of 60 seconds.

frontend/src/context/event-theme-context.tsx

@@ -98,7 +98,7 @@ export const EventThemesProvider: React.FC<EventThemesProviderProps> = ({
   } = useQuery({
     ...listEventThemesOptions(),
     refetchOnWindowFocus: "always",
-    refetchIntervalInBackground: true,
+    refetchInterval: 60000,
   });
 
   // Fetch specific theme