feat: add showScrollbar prop to VirtualGrid (#7227)

## Summary

Enable vertical scrollbar in Media Assets sidebar for better navigation
when content overflows.

The original implementation hid the scrollbar by default. 
Since the intent behind hiding it wasn't documented, this change
preserves the default behavior (showScrollbar: false) while allowing
components to opt-in to visible scrollbars when needed.

fix https://github.com/Comfy-Org/ComfyUI_frontend/issues/6914

## Screenshots (if applicable)
<img width="681" height="890" alt="image"
src="https://github.com/user-attachments/assets/af48a440-6d04-4226-9482-eb17f8d11a40"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7227-feat-add-showScrollbar-prop-to-VirtualGrid-2c36d73d365081c8955ee632c6c644f7)
by [Unito](https://www.unito.io)

src/components/common/VirtualGrid.vue

@@ -117,16 +117,7 @@ onBeforeUnmount(() => {
 .scroll-container {
   height: 100%;
   overflow-y: auto;
-
-  /* Firefox */
-  scrollbar-width: none;
-
-  &::-webkit-scrollbar {
-    width: 1px;
-  }
-
-  &::-webkit-scrollbar-thumb {
-    background-color: transparent;
-  }
+  scrollbar-width: thin;
+  scrollbar-color: var(--dialog-surface) transparent;
 }
 </style>