[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/locales/en/main.json

@@ -181,6 +181,7 @@
     "missing": "Missing",
     "inProgress": "In progress",
     "completed": "Completed",
+    "downloading": "Downloading",
     "interrupted": "Interrupted",
     "queued": "Queued",
     "running": "Running",
@@ -2286,14 +2287,16 @@
     "noAssetsFound": "No assets found",
     "noModelsInFolder": "No {type} available in this folder",
     "notSureLeaveAsIs": "Not sure? Just leave this as is",
+    "noValidSourceDetected": "No valid import source detected",
     "onlyCivitaiUrlsSupported": "Only Civitai URLs are supported",
     "ownership": "Ownership",
     "ownershipAll": "All",
     "ownershipMyModels": "My models",
     "ownershipPublicModels": "Public models",
+    "processingModel": "Download started",
+    "processingModelDescription": "You can close this dialog. The download will continue in the background.",
     "providerCivitai": "Civitai",
     "providerHuggingFace": "Hugging Face",
-    "noValidSourceDetected": "No valid import source detected",
     "selectFrameworks": "Select Frameworks",
     "selectModelType": "Select model type",
     "selectProjects": "Select Projects",
@@ -2318,8 +2321,8 @@
     "uploadModelDescription1": "Paste a Civitai model download link to add it to your library.",
     "uploadModelDescription1Generic": "Paste a model download link to add it to your library.",
     "uploadModelDescription2": "Only links from {link} are supported at the moment",
-    "uploadModelDescription2Link": "https://civitai.com/models",
     "uploadModelDescription2Generic": "Only URLs from the following providers are supported:",
+    "uploadModelDescription2Link": "https://civitai.com/models",
     "uploadModelDescription3": "Max file size: {size}",
     "uploadModelFailedToRetrieveMetadata": "Failed to retrieve metadata. Please check the link and try again.",
     "uploadModelFromCivitai": "Import a model from Civitai",
@@ -2343,6 +2346,11 @@
       "complete": "{assetName} has been deleted.",
       "failed": "{assetName} could not be deleted."
     },
+    "download": {
+      "complete": "Download complete",
+      "failed": "Download failed",
+      "inProgress": "Downloading {assetName}..."
+    },
     "rename": {
       "failed": "Could not rename asset."
     }