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/scripts/utils.ts

@@ -51,20 +51,8 @@ export async function addStylesheet(
   })
 }
 
-export function downloadBlob(filename: string, blob: Blob) {
-  const url = URL.createObjectURL(blob)
-  const a = $el('a', {
-    href: url,
-    download: filename,
-    style: { display: 'none' },
-    parent: document.body
-  })
-  a.click()
-  setTimeout(function () {
-    a.remove()
-    window.URL.revokeObjectURL(url)
-  }, 0)
-}
+/** @knipIgnoreUnusedButUsedByCustomNodes */
+export { downloadBlob } from '@/base/common/downloadUtil'
 
 export function uploadFile(accept: string) {
   return new Promise<File>((resolve, reject) => {