refactor: simplify asset download state and fix deletion UI (#7974)

## Summary

Refactors asset download state management and fixes asset deletion UI
issues.

## Changes

### assetDownloadStore simplification
- Replace `pendingModelTypes` Map with `modelType` stored directly on
`AssetDownload`
- Replace `completedDownloads` array with single `lastCompletedDownload`
ref
- `trackDownload()` now creates a placeholder entry immediately
- Use VueUse `whenever` instead of `watch` for cleaner null handling

### Asset refresh on download completion
- Refresh all relevant caches when a download completes:
  - Node type caches (e.g., "CheckpointLoaderSimple")
  - Tag caches (e.g., "tag:checkpoints")
  - "All Models" cache ("tag:models")

### Asset deletion fix
- Remove local `deletedLocal` state that caused blank grid cells
- Emit `deleted` event from AssetCard → AssetGrid → AssetBrowserModal
- Trigger store refresh on deletion to properly remove the asset from
the grid

## Testing

- Added test for out-of-order websocket message handling
- All existing tests pass

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7974-refactor-simplify-asset-download-state-and-fix-deletion-UI-2e76d73d365081c69bcde9150a0d460c)
by [Unito](https://www.unito.io)

---------

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

src/platform/assets/composables/useUploadModelWizard.ts

@@ -245,7 +245,8 @@ export function useUploadModelWizard(modelTypes: Ref<ModelTypeOption[]>) {
           if (selectedModelType.value) {
             assetDownloadStore.trackDownload(
               result.task.task_id,
-              selectedModelType.value
+              selectedModelType.value,
+              filename
             )
           }
           uploadStatus.value = 'processing'