fix(downloads): Retry draws the refresh glyph, not Play (E28 S4)

The first UI-review run (Downloads) found a failed row whose Retry wore the
Play glyph in the row and its menu, promising a video. DESIGN.md gains
'a glyph means one thing' and a log row.

frontend/src/components/DownloadCenter.tsx

@@ -9,6 +9,7 @@ import {
   Play,
   PlayCircle,
   Plus,
+  RefreshCw,
   Settings2,
   Share2,
   UploadCloud,
@@ -225,7 +226,12 @@ export default function DownloadCenter() {
                           : "downloads.actions.resume",
                       )}
                     >
-                      <Play className="w-4 h-4" />
+                      {/* Retry is "do it again", not playback: the Play glyph promised a video. */}
+                      {job.status === "error" ? (
+                        <RefreshCw className="w-4 h-4" />
+                      ) : (
+                        <Play className="w-4 h-4" />
+                      )}
                     </IconBtn>
                   )}
                 </DownloadRow>