YT: keep yt-dlp's last error line (logged + shown) instead of a bare 'Download failed'

ddiff --git a/apps/yt/server/downloader.js b/apps/yt/server/downloader.js
iindex 64adb9a..616b448 100644
---- a/apps/yt/server/downloader.js
++++ b/apps/yt/server/downloader.js
@@@ -152,6 +152,19 @@ function getAvailableFormats(info) {
      return formats;
  }
  
++/** yt-dlp's last stderr line → something the person can act on (the raw line is logged). */
++function friendlyDownloadError(line) {
++    const t = String(line || '');
++    if (/Sign in to confirm|not a bot|cookies/i.test(t)) return 'YouTube is asking this server to prove it is not a bot — try again in a few minutes';
++    if (/Private video|members-only|login required/i.test(t)) return 'This video is private or members-only';
++    if (/Video unavailable|has been removed|not available/i.test(t)) return 'This video is unavailable';
++    if (/age|confirm your age/i.test(t)) return 'Age-restricted videos cannot be downloaded';
++    if (/ffmpeg|ffprobe|Postprocessing/i.test(t)) return `Conversion failed on the server (${t.slice(0, 120)})`;
++    if (/Requested format is not available/i.test(t)) return 'That quality is not available for this video — try another';
++    if (/HTTP Error 4\d\d/i.test(t)) return `YouTube refused the request (${t.match(/HTTP Error \d+/)[0]})`;
++    return t.replace(/^ERROR:\s*/i, '').slice(0, 160) || 'Download failed';
++}
++
  // ── Download Video ───────────────────────────────────────────
  /**
   * Start a download and return a tracking ID.
@@@ -222,8 +235,10 @@ function startDownload(url, quality = 'best') {
              }
          });
  
++        let stderrTail = '';
          proc.stderr.on('data', (data) => {
              const text = data.toString();
++            stderrTail = (stderrTail + text).slice(-2000);
              // Some progress info goes to stderr too
              const match = text.match(/([\d.]+)%/);
              if (match) entry.progress = parseFloat(match[1]);
@@@ -232,8 +247,10 @@ function startDownload(url, quality = 'best') {
          proc.on('close', (code) => {
              currentConcurrent--;
              if (code !== 0) {
++                const last = stderrTail.split('\n').map(l => l.trim()).filter(l => l && !/^\[download\]/.test(l)).pop() || `yt-dlp exited with code ${code}`;
++                console.error(`[Download] ${id} (${quality}) failed: ${last}`);
                  entry.status = 'error';
--                entry.error = 'Download failed';
++                entry.error = friendlyDownloadError(last);
                  return;
              }