fix: virtual scroll pagination not working in media asset list view (#9646)

## Summary

Fix virtual scroll pagination not triggering in media asset panel list
view.

## Changes

**What**: `VirtualGrid` in `AssetsSidebarListView` was missing
`maxColumns=1` and had an incorrect default item height (200px vs actual
~48px). Without `maxColumns`, `cols` was calculated as
`floor(containerWidth / 200)` (e.g. 2), causing the row count to be
halved and `isNearEnd` to never fire correctly. Added `:max-columns="1"`
and `:default-item-height="48"` to fix pagination. Added regression
tests to `VirtualGrid.test.ts`.

## Review Focus

The root cause: `VirtualGrid.cols` computed as `floor(width/200)`
instead of `1` for single-column list layout, breaking spacer heights
and `approach-end` detection.

Test covers both the fix (approach-end fires with maxColumns=1) and the
bug reproduction (does not fire without it).

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9646-fix-virtual-scroll-pagination-not-working-in-media-asset-list-view-31e6d73d3650813d973ad19638ad6933)
by [Unito](https://www.unito.io)

src/components/sidebar/tabs/AssetsSidebarListView.vue

@@ -18,6 +18,8 @@
       class="flex-1"
       :items="assetItems"
       :grid-style="listGridStyle"
+      :max-columns="1"
+      :default-item-height="48"
       @approach-end="emit('approach-end')"
     >
       <template #item="{ item }">