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/layout/Sidebar.tsx

@@ -19,9 +19,24 @@ interface NavDef {
 
 const MAIN_NAV: NavDef[] = [
   { to: '/library', labelKey: 'nav.library', icon: 'vinyl-record' },
-  { to: '/discover', labelKey: 'nav.search', icon: 'magnifying-glass', perm: 'download' },
-  { to: '/downloads', labelKey: 'nav.downloads', icon: 'arrow-circle-down', perm: 'download' },
-  { to: '/upload', labelKey: 'nav.upload', icon: 'upload-simple', perm: 'upload' },
+  {
+    to: '/discover',
+    labelKey: 'nav.search',
+    icon: 'magnifying-glass',
+    perm: 'download',
+  },
+  {
+    to: '/downloads',
+    labelKey: 'nav.downloads',
+    icon: 'arrow-circle-down',
+    perm: 'download',
+  },
+  {
+    to: '/upload',
+    labelKey: 'nav.upload',
+    icon: 'upload-simple',
+    perm: 'upload',
+  },
   { to: '/storage', labelKey: 'nav.storage', icon: 'hard-drives' },
 ];