fix(download): warn before the tab closes mid-download

Bunny and direct downloads are pulled through fetch() so we can save them
under our own filename. The browser does not treat that as a download, so
closing the tab discarded everything received so far without a word.

Register a reference counted beforeunload guard while those transfers are
in flight, and while a project manifest is being pulled file by file.
Browser owned downloads (same-origin proxy, the over-10GB fallback, asset
downloads) survive a tab close on their own and stay unguarded.

components/video-page/hooks/use-download-actions.ts

@@ -22,6 +22,7 @@ import {
   createDownloadProgressToast,
   type DownloadProgressToastHandle,
 } from '@/components/download-progress-toast';
+import { beginUnloadGuard } from '@/lib/client/unload-guard';
 
 function sanitizeDownloadFileName(value: string): string {
   return value
@@ -94,6 +95,7 @@ export function useDownloadActions({ activeVersion, video }: UseDownloadActionsP
       isDownloadingRef.current = true;
       setActiveDownloadTarget(target);
       let progressToast: DownloadProgressToastHandle | null = null;
+      let releaseUnloadGuard: (() => void) | null = null;
       try {
         let downloadUrl: string | null = null;
 
@@ -167,6 +169,9 @@ export function useDownloadActions({ activeVersion, video }: UseDownloadActionsP
             title: `Downloading “${baseName}”`,
             description: 'Starting…',
           });
+          // The bytes only exist in this tab until the blob is saved, so warn
+          // before the page goes away instead of losing the whole transfer.
+          releaseUnloadGuard = beginUnloadGuard();
           const saved = await downloadNamedFile(downloadUrl, `${baseName}.${fallbackExt}`, (p) => {
             progressToast?.update({
               description: downloadProgressLabel(p),
@@ -195,6 +200,7 @@ export function useDownloadActions({ activeVersion, video }: UseDownloadActionsP
           toast.error('Failed to start download');
         }
       } finally {
+        releaseUnloadGuard?.();
         isDownloadingRef.current = false;
         setActiveDownloadTarget(null);
       }