refactor: centralize all download utils across app and apply special cloud-specific behavior (#6188)

## Summary

Centralized all download functionalities across app. Then changed
downloadFile on the cloud distribution to stream assets via blob fetches
while desktop/local retains direct anchor downloads. This fixes issue
where trying to download cross-origin resources opens them in the
window, potentially losing the user's unsaved changes.

## Changes

- **What**: Moved `downloadBlob` into `downloadUtil`, routed all callers
(3D exporter, recording manager, node template export, workflow/palette
export, Litegraph save, ~~`useDownload` consumers~~) through shared
helpers, and changed `downloadFile` to `fetch` first when `isCloud` so
cross-origin URLs download reliably
- `useDownload` is the exception since we simply cannot do model
downloads through blob (forcing user to transfer the entire model data
twice is bad). Fortunately on cloud, the user doesn't need to download
models locally anyway.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6188-refactor-centralize-all-download-utils-across-app-and-apply-special-cloud-specific-behav-2946d73d365081de9f27f0994950511d)
by [Unito](https://www.unito.io)

src/base/common/downloadUtil.ts

@@ -1,29 +1,64 @@
 /**
  * Utility functions for downloading files
  */
+import { isCloud } from '@/platform/distribution/types'
 
 // Constants
 const DEFAULT_DOWNLOAD_FILENAME = 'download.png'
 
+/**
+ * Trigger a download by creating a temporary anchor element
+ * @param href - The URL or blob URL to download
+ * @param filename - The filename to suggest to the browser
+ */
+function triggerLinkDownload(href: string, filename: string): void {
+  const link = document.createElement('a')
+  link.href = href
+  link.download = filename
+  link.style.display = 'none'
+
+  document.body.appendChild(link)
+  link.click()
+  document.body.removeChild(link)
+}
+
 /**
  * Download a file from a URL by creating a temporary anchor element
  * @param url - The URL of the file to download (must be a valid URL string)
  * @param filename - Optional filename override (will use URL filename or default if not provided)
  * @throws {Error} If the URL is invalid or empty
  */
-export const downloadFile = (url: string, filename?: string): void => {
+export function downloadFile(url: string, filename?: string): void {
   if (!url || typeof url !== 'string' || url.trim().length === 0) {
     throw new Error('Invalid URL provided for download')
   }
-  const link = document.createElement('a')
-  link.href = url
-  link.download =
+
+  const inferredFilename =
     filename || extractFilenameFromUrl(url) || DEFAULT_DOWNLOAD_FILENAME
 
-  // Trigger download
-  document.body.appendChild(link)
-  link.click()
-  document.body.removeChild(link)
+  if (isCloud) {
+    // Assets from cross-origin (e.g., GCS) cannot be downloaded this way
+    void downloadViaBlobFetch(url, inferredFilename).catch((error) => {
+      console.error('Failed to download file', error)
+    })
+    return
+  }
+
+  triggerLinkDownload(url, inferredFilename)
+}
+
+/**
+ * Download a Blob by creating a temporary object URL and anchor element
+ * @param filename - The filename to suggest to the browser
+ * @param blob - The Blob to download
+ */
+export function downloadBlob(filename: string, blob: Blob): void {
+  const url = URL.createObjectURL(blob)
+
+  triggerLinkDownload(url, filename)
+
+  // Revoke on the next microtask to give the browser time to start the download
+  queueMicrotask(() => URL.revokeObjectURL(url))
 }
 
 /**
@@ -39,3 +74,15 @@ const extractFilenameFromUrl = (url: string): string | null => {
     return null
   }
 }
+
+const downloadViaBlobFetch = async (
+  href: string,
+  filename: string
+): Promise<void> => {
+  const response = await fetch(href)
+  if (!response.ok) {
+    throw new Error(`Failed to fetch ${href}: ${response.status}`)
+  }
+  const blob = await response.blob()
+  downloadBlob(filename, blob)
+}