fix: show skeleton loading state in asset folder view (#8979)

## Description

When clicking a multi-output job to enter folder view,
`resolveOutputAssetItems` fetches job details asynchronously. During
this fetch, the panel showed "No generated files found" because there
was no loading state for the folder resolution—only the media list fetch
had one.

This replaces the empty state flash with skeleton cards that match the
asset grid layout, using the known output count from metadata to render
the correct number of placeholders.

Supersedes #8960.

### Changes

- **Add shadcn/vue `Skeleton` component**
(`src/components/ui/skeleton/Skeleton.vue`)
- **Use `useAsyncState`** from VueUse to track folder asset resolution,
providing `isLoading` automatically
- **Wire `folderLoading`** into `showLoadingState` and `showEmptyState`
computeds
- **Replace `ProgressSpinner`** with a skeleton grid that mirrors the
asset card layout
- **Use `metadata.outputCount`** to predict skeleton count; falls back
to 6

### Before / After

| Before | After |
|--------|-------|
| "No generated files found" flash | Skeleton cards matching grid layout
|

## Checklist

- [x] Code follows project conventions
- [x] No `any` types introduced
- [x] Lint and typecheck pass

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8979-fix-show-skeleton-loading-state-in-asset-folder-view-30c6d73d365081fa9809f616204ed234)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>

src/components/ui/skeleton/Skeleton.vue

@@ -0,0 +1,15 @@
+<script setup lang="ts">
+import type { HTMLAttributes } from 'vue'
+
+import { cn } from '@/utils/tailwindUtil'
+
+const { class: className } = defineProps<{
+  class?: HTMLAttributes['class']
+}>()
+</script>
+
+<template>
+  <div
+    :class="cn('animate-pulse rounded-md bg-secondary-background', className)"
+  />
+</template>