Translate yt-dlp errors and drop the redundant prefix hint

Errors reached people as raw tool output: "Unsupported URL:
https://...%D0%B0%D1%83..." says nothing to whoever pasted a link. The
common cases now map to plain descriptions with the next step where there
is one (age gate and bot checks point at YTDLP_COOKIES, a stale extractor
at updating yt-dlp), URLs are decoded, and anything unrecognised still
falls through verbatim.

The panel's prefix hint is gone now that the source picker does the same
job visibly; prefixes keep working and stay documented for chat.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/src/components/SearchPanel.tsx

@@ -12,6 +12,7 @@ const SOURCE_BADGE: Record<Track["source"], string> = {
 interface Props {
   serverId: string;
   canControl: boolean;
+  /** Adds the local library to the source picker. */
   localLibrary: boolean;
   onError(message: string | null): void;
 }
@@ -126,16 +127,6 @@ export function SearchPanel({ serverId, canControl, localLibrary, onError }: Pro
           ))}
         </ul>
       )}
-
-      <p className="hint">
-        Префиксы: <code>sc:</code> — искать в SoundCloud, <code>yt:</code> — в YouTube
-        {localLibrary ? (
-          <>
-            , <code>local:</code> — в локальной медиатеке
-          </>
-        ) : null}
-        .
-      </p>
     </div>
   );
 }