Explain save failures and keep download diagnostics available

frontend/downloads-page.js

@@ -1,6 +1,7 @@
 /* Downloads page UI module. Grouped jobs, storage info, confirmed cancellation. */
 (function (root) {
   'use strict';
+  const errors = typeof module !== 'undefined' ? require('./download-errors.js') : root.DownloadErrors;
 
   const bytes = n => {
     if (typeof n !== 'number' || isNaN(n) || n <= 0) return '0 MB';
@@ -18,7 +19,7 @@
 
   function formatJobStatus(job) {
     if (job.status === 'failed') {
-      return job.error ? `Failed: ${job.error}` : 'Save failed';
+      return job.error ? `Failed: ${errors.message(job.error)}` : 'Save failed';
     }
     if (job.status === 'paused') {
       return 'Paused — resume when you’re ready';
@@ -215,6 +216,8 @@
           </div>
         `;
 
+        if (isFailed && job.error) errors.appendDetails(row.querySelector('.card-info'), job.error);
+
         const acts = doc.createElement('div');
         acts.className = 'dl-actions';