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/features/library/LibraryPage.tsx

@@ -76,13 +76,25 @@ export function LibraryPage() {
   // Live server data wins; offline we fall back to the locally-composed list.
   const tracksToShow =
     tracksQuery.data?.items ??
-    (offline ? (q ? localTracks.filter((tr) => matchTrack(tr, q)) : localTracks) : undefined);
+    (offline
+      ? q
+        ? localTracks.filter((tr) => matchTrack(tr, q))
+        : localTracks
+      : undefined);
   const albumsToShow =
     albumsQuery.data?.items ??
-    (offline ? (q ? localAlbums.filter((a) => matchAlbum(a, q)) : localAlbums) : undefined);
+    (offline
+      ? q
+        ? localAlbums.filter((a) => matchAlbum(a, q))
+        : localAlbums
+      : undefined);
   const artistsToShow =
     artistsQuery.data?.items ??
-    (offline ? (q ? localArtists.filter((a) => matchArtist(a, q)) : localArtists) : undefined);
+    (offline
+      ? q
+        ? localArtists.filter((a) => matchArtist(a, q))
+        : localArtists
+      : undefined);
 
   const handlePlayAll = (tracks: Track[]) => {
     dispatch(