load assets browser before fetch completes and show loading state (#6189)

## Summary

Moves the fetch and post-fetch logic associated with the asset browser
into the component and shows a loading state while fetching.

To test, use this branch:
https://github.com/comfyanonymous/ComfyUI/pull/10045



https://github.com/user-attachments/assets/718974d5-efc7-46a0-bcd6-e82596d4c389

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6189-load-assets-browser-before-fetch-completes-and-show-loading-state-2946d73d365081879d1bd05d86e8c036)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>

src/platform/assets/components/AssetGrid.vue

@@ -37,12 +37,12 @@
     <!-- Loading state -->
     <div
       v-if="loading"
-      class="col-span-full flex items-center justify-center py-16"
+      class="col-span-full flex items-center justify-center py-20"
     >
       <i
         class="icon-[lucide--loader]"
         :class="
-          cn('size-6 animate-spin', 'text-stone-300 dark-theme:text-stone-200')
+          cn('size-12 animate-spin', 'text-stone-300 dark-theme:text-stone-200')
         "
       />
     </div>