feat: name video downloads by title + version

Downloads now save as "<video title> <version label>" (or "<title> vN"
when no label), with the real extension derived from the file's content
type, instead of the CDN's generic "original" name.

- Bunny (cross-origin CDN redirect) files are fetched and saved as a named
  blob, but only up to 10 GB; larger files fall back to a plain navigation
  so the browser streams to disk without buffering in memory.
- R2 / S3 / MinIO uploads are same-origin (/api/upload/video/...), so the
  download attribute names them correctly at any size, no buffering.
- Applies to both single-video and bulk/project downloads; bulk downloads
  run sequentially so at most one file is buffered at a time.
- Shared helper in lib/client/download-file.ts.

lib/client/project-download.ts

@@ -1,5 +1,7 @@
 'use client';
 
+import { downloadNamedFile, navigateDownload } from '@/lib/client/download-file';
+
 const DOWNLOAD_STAGGER_MS = 500;
 
 export type ProjectDownloadManifestFile = {
@@ -19,21 +21,28 @@ function sleep(ms: number): Promise<void> {
   return new Promise((resolve) => window.setTimeout(resolve, ms));
 }
 
-function triggerBrowserDownload(file: ProjectDownloadManifestFile): void {
-  const anchor = document.createElement('a');
-  anchor.href = file.url;
-  anchor.rel = 'noopener';
-  if (file.url.startsWith('/')) {
-    anchor.download = file.fileName;
+async function triggerBrowserDownload(file: ProjectDownloadManifestFile): Promise<void> {
+  // Same-origin proxy files (R2 / S3 / MinIO via /api/upload/video/...): the
+  // download attribute applies and the browser streams straight to disk, so the
+  // name is correct at any size with no memory cost.
+  if (file.url.startsWith('/api/upload/video/')) {
+    navigateDownload(file.url, file.fileName);
+    return;
+  }
+
+  // Bunny (CDN redirect) and external direct hosts are cross-origin, so the name
+  // only applies if we fetch the bytes. downloadNamedFile does that for files up
+  // to 10 GB; larger ones fall back to a plain navigation (CDN filename).
+  const saved = await downloadNamedFile(file.url, file.fileName);
+  if (!saved) {
+    navigateDownload(file.url, file.fileName);
   }
-  document.body.appendChild(anchor);
-  anchor.click();
-  anchor.remove();
 }
 
 export async function runProjectDownloadManifest(manifest: ProjectDownloadManifest): Promise<void> {
   for (let index = 0; index < manifest.files.length; index += 1) {
-    triggerBrowserDownload(manifest.files[index]!);
+    // Sequential so at most one file is buffered in memory at a time.
+    await triggerBrowserDownload(manifest.files[index]!);
     if (index < manifest.files.length - 1) {
       await sleep(DOWNLOAD_STAGGER_MS);
     }