[feat] Add async model upload with WebSocket progress tracking (#7746)

## Summary
- Adds asynchronous model upload support with HTTP 202 responses
- Implements WebSocket-based real-time download progress tracking via
`asset_download` events
- Creates `assetDownloadStore` for centralized download state management
and toast notifications
- Updates upload wizard UI to show "processing" state when downloads
continue in background

## Changes
- **Core**: New `assetDownloadStore` for managing async downloads with
WebSocket events
- **API**: Support for HTTP 202 async upload responses with task
tracking
- **UI**: Upload wizard now shows "processing" state and allows closing
dialog during download
- **Progress**: Periodic toast notifications (every 5s) during active
downloads with completion/error toasts
- **Schema**: Updated task statuses (`created`, `running`, `completed`,
`failed`) and WebSocket message types

## Review Focus
- WebSocket event handling and download state management in
`assetDownloadStore`
- Upload flow UX - users can now close the dialog and download continues
in background
- Toast notification frequency and timing
- Schema alignment with backend async upload API

Fixes #7748

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7746-feat-Add-async-model-upload-with-WebSocket-progress-tracking-2d36d73d3650811cb79ae06f470dcded)
by [Unito](https://www.unito.io)

---------

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

src/scripts/api.ts

@@ -17,6 +17,7 @@ import type {
   NodeId
 } from '@/platform/workflow/validation/schemas/workflowSchema'
 import type {
+  AssetDownloadWsMessage,
   EmbeddingsResponse,
   ExecutedWsMessage,
   ExecutingWsMessage,
@@ -153,6 +154,7 @@ interface BackendApiCalls {
   progress_text: ProgressTextWsMessage
   progress_state: ProgressStateWsMessage
   feature_flags: FeatureFlagsWsMessage
+  asset_download: AssetDownloadWsMessage
 }
 
 /** Dictionary of all api calls */
@@ -664,6 +666,7 @@ export class ComfyApi extends EventTarget {
             case 'logs':
             case 'b_preview':
             case 'notification':
+            case 'asset_download':
               this.dispatchCustomEvent(msg.type, msg.data)
               break
             case 'feature_flags':