feat(discover): wire A4 search + A5 downloads to backend

Adds DownloadJob/ExternalSearchResult/SourceInfo contract types + mappers, the downloads + search RTKQ endpoints, and the SearchDownloadPage (search external sources, per-result download states) and DownloadsManagerPage (active/history, progress, retry/cancel, poll-while-active). en/ru i18n. Snapshot also bundles in-progress queue/metadata-editor/storage UI work.

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

src/components/player/QueuePanel.tsx

@@ -273,7 +273,10 @@ function QueueRow({
       </div>
       <div className="qt">
         <Marquee className="t" text={resolved?.title ?? entry.title} />
-        <Marquee className="r" text={resolved?.artistName ?? entry.artistName} />
+        <Marquee
+          className="r"
+          text={resolved?.artistName ?? entry.artistName}
+        />
       </div>
       <Menu>
         <MenuTrigger asChild>