download job: use the panel's exact url list (no server re-expansion)

A YouTube Mix/radio playlist (list=RD…) returns a different set of items on every
/api/playlist call (observed 779 / 1485 / 529 for the same URL). The job used to
re-expand the playlist server-side, so it would download a different list than the
count shown on the decision screen.

The panel now passes the already-expanded `urls[]` into the job, and the executor
uses them verbatim (falling back to expanding `url` only when no list is given).
The job downloads exactly what you decided on. Endpoint takes `urls[]` (stored as
inputs.urls) or `url`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/workspaces/officerdev/src/apps/FileBrowser/VideoDownloadPanel.tsx

@@ -250,7 +250,7 @@ export const VideoDownloadPanel = () => {
   const startJob = async () => {
     try {
       const res = await client.post<{ jobId: string; status: string }>('/jobs/download', {
-        url: url.trim(),
+        urls: expandedUrls, // the exact list we counted — a Mix playlist won't re-expand to a different set
         format: jobFormat,
         dir: targetDir(),
         root,