feat: implement progressive pagination for Asset Browser model assets (#8212)

## Summary

Implements progressive pagination for model assets - returns the first
batch immediately while loading remaining batches in the background.

## Changes

### Store (`assetsStore.ts`)
- Adds `ModelPaginationState` tracking (assets Map, offset, hasMore,
loading, error)
- `updateModelsForKey()` returns first batch, then calls
`loadRemainingBatches()` to fetch the rest
- Accessor functions `getAssets(key)`, `isModelLoading(key)` replace
direct Map access

### API (`assetService.ts`)
- Adds `PaginationOptions` interface (`{ limit?, offset? }`)

### Components
- `AssetBrowserModal.vue` uses new accessor API

### Tests
- Updated mocks for new accessor pattern

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8212-feat-implement-progressive-pagination-for-Asset-Browser-model-assets-2ef6d73d36508157af04d1264780997e)
by [Unito](https://www.unito.io)

---------

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

src/renderer/extensions/vueNodes/widgets/composables/useAssetWidgetData.desktop.test.ts

@@ -12,9 +12,9 @@ const mockGetCategoryForNodeType = vi.fn()
 
 vi.mock('@/stores/assetsStore', () => ({
   useAssetsStore: () => ({
-    modelAssetsByNodeType: new Map(),
-    modelLoadingByNodeType: new Map(),
-    modelErrorByNodeType: new Map(),
+    getAssets: () => [],
+    isModelLoading: () => false,
+    getError: () => undefined,
     updateModelsForNodeType: mockUpdateModelsForNodeType
   })
 }))