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/download-file.ts

@@ -0,0 +1,92 @@
+'use client';
+
+// Above this size we don't buffer the file in memory to rename it — the caller
+// falls back to a plain navigation so the browser streams it straight to disk
+// (with the CDN's own filename). 10 GiB.
+export const MAX_NAMED_DOWNLOAD_BYTES = 10 * 1024 * 1024 * 1024;
+
+const MIME_EXTENSION_MAP: Record<string, string> = {
+  'video/mp4': 'mp4',
+  'video/webm': 'webm',
+  'video/quicktime': 'mov',
+  'video/x-matroska': 'mkv',
+  'video/x-msvideo': 'avi',
+};
+
+export function extensionFromUrl(url: string): string {
+  const path = url.split('?')[0] ?? url;
+  const dot = path.lastIndexOf('.');
+  if (dot === -1) return '';
+  const ext = path.slice(dot + 1).toLowerCase();
+  return ext.length >= 1 && ext.length <= 5 ? ext : '';
+}
+
+function replaceExtension(fileName: string, ext: string): string {
+  const dot = fileName.lastIndexOf('.');
+  const stem = dot > 0 ? fileName.slice(0, dot) : fileName;
+  return `${stem}.${ext}`;
+}
+
+export function saveBlobAs(blob: Blob, fileName: string): void {
+  const objectUrl = URL.createObjectURL(blob);
+  const anchor = document.createElement('a');
+  anchor.href = objectUrl;
+  anchor.download = fileName;
+  document.body.appendChild(anchor);
+  anchor.click();
+  anchor.remove();
+  // Revoke after the download has had a chance to start.
+  setTimeout(() => URL.revokeObjectURL(objectUrl), 10_000);
+}
+
+/**
+ * Fetch a (possibly cross-origin) file and save it under `fileName`. The
+ * browser's `download` attribute is ignored across origins / redirects, so for
+ * CDN sources we pull the bytes (CORS is open on them) and save the blob, which
+ * lets us control the name and derive the real extension from the content type.
+ *
+ * Returns `false` (without downloading) when the file is larger than
+ * MAX_NAMED_DOWNLOAD_BYTES — buffering that in memory would be unsafe — or when
+ * the fetch isn't usable, so the caller can fall back to a plain navigation.
+ * Returns `true` when the named blob was saved.
+ */
+export async function downloadNamedFile(url: string, fileName: string): Promise<boolean> {
+  let res: Response;
+  try {
+    res = await fetch(url, { cache: 'no-store' });
+  } catch {
+    return false;
+  }
+  if (!res.ok) return false;
+
+  const contentLength = Number(res.headers.get('content-length'));
+  if (Number.isFinite(contentLength) && contentLength > MAX_NAMED_DOWNLOAD_BYTES) {
+    await res.body?.cancel().catch(() => {});
+    return false;
+  }
+
+  let blob: Blob;
+  try {
+    blob = await res.blob();
+  } catch {
+    return false;
+  }
+
+  const mimeExt = MIME_EXTENSION_MAP[blob.type];
+  saveBlobAs(blob, mimeExt ? replaceExtension(fileName, mimeExt) : fileName);
+  return true;
+}
+
+/** Plain navigation download (streams to disk; filename controlled only for
+ * same-origin URLs via the download attribute). */
+export function navigateDownload(url: string, sameOriginFileName?: string): void {
+  const anchor = document.createElement('a');
+  anchor.href = url;
+  anchor.rel = 'noopener';
+  if (sameOriginFileName && url.startsWith('/')) {
+    anchor.download = sameOriginFileName;
+  }
+  document.body.appendChild(anchor);
+  anchor.click();
+  anchor.remove();
+}