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>

tests/sw-core.test.ts

@@ -7,9 +7,9 @@ import {
 } from '../public/sw-core.js';
 
 test('trackIdFromUrl extracts the content id from a stream URL', () => {
-  expect(
-    trackIdFromUrl('https://host/api/v1/stream/abc123?token=xyz'),
-  ).toBe('abc123');
+  expect(trackIdFromUrl('https://host/api/v1/stream/abc123?token=xyz')).toBe(
+    'abc123',
+  );
   expect(trackIdFromUrl('https://host/api/v1/library/albums')).toBeNull();
 });