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.ts

@@ -34,23 +34,17 @@ export function useAssetWidgetData(
 
     const assets = computed<AssetItem[]>(() => {
       const resolvedType = toValue(nodeType)
-      return resolvedType
-        ? (assetsStore.modelAssetsByNodeType.get(resolvedType) ?? [])
-        : []
+      return resolvedType ? (assetsStore.getAssets(resolvedType) ?? []) : []
     })
 
     const isLoading = computed(() => {
       const resolvedType = toValue(nodeType)
-      return resolvedType
-        ? (assetsStore.modelLoadingByNodeType.get(resolvedType) ?? false)
-        : false
+      return resolvedType ? assetsStore.isModelLoading(resolvedType) : false
     })
 
     const error = computed<Error | null>(() => {
       const resolvedType = toValue(nodeType)
-      return resolvedType
-        ? (assetsStore.modelErrorByNodeType.get(resolvedType) ?? null)
-        : null
+      return resolvedType ? (assetsStore.getError(resolvedType) ?? null) : null
     })
 
     const dropdownItems = computed<DropdownItem[]>(() => {
@@ -71,7 +65,8 @@ export function useAssetWidgetData(
           return
         }
 
-        const hasData = assetsStore.modelAssetsByNodeType.has(currentNodeType)
+        const existingAssets = assetsStore.getAssets(currentNodeType) ?? []
+        const hasData = existingAssets.length > 0
 
         if (!hasData) {
           await assetsStore.updateModelsForNodeType(currentNodeType)