refactor(api): F.50 — extract 7 small domain modules (#615)

Seven domain-eng splits peel ~200 LOC of read endpoints out of
`api/subsonic.ts`:

- `subsonicStreamUrl.ts` — `buildStreamUrl`, `coverArtCacheKey`,
  `buildCoverArtUrl`, `buildDownloadUrl` (token-signed URL builders
  for the four /rest endpoints we hand to the browser).
- `subsonicStarRating.ts` — `getStarred`, `star`, `unstar`,
  `setRating`, `probeEntityRatingSupport`. `setRating` still triggers
  the lazy `navidromeBrowse` cache invalidation; the same-folder
  lazy import path is preserved.
- `subsonicSearch.ts` — `search`, `searchSongsPaged`.
- `subsonicScrobble.ts` — `scrobbleSong`, `reportNowPlaying`,
  `getNowPlaying`.
- `subsonicAlbumInfo.ts` — `getAlbumInfo2`.
- `subsonicLyrics.ts` — `getLyricsBySongId`.
- `subsonicGenres.ts` — `getGenres`, `getAlbumsByGenre`.

63 external call sites migrated to direct imports. Four `vi.mock`
targets in the store-level tests pointed at `../api/subsonic` and
were updated to the new module paths.

Pure code-move. subsonic.ts: 762 → 561 LOC (−201).

src/api/subsonicSearch.ts

@@ -0,0 +1,43 @@
+import { api, libraryFilterParams } from './subsonicClient';
+import type {
+  SearchResults,
+  SubsonicAlbum,
+  SubsonicArtist,
+  SubsonicSong,
+} from './subsonicTypes';
+
+export async function search(query: string, options?: { albumCount?: number; artistCount?: number; songCount?: number }): Promise<SearchResults> {
+  if (!query.trim()) return { artists: [], albums: [], songs: [] };
+  const data = await api<{
+    searchResult3: {
+      artist?: SubsonicArtist[];
+      album?: SubsonicAlbum[];
+      song?: SubsonicSong[];
+    };
+  }>('search3.view', {
+    query,
+    artistCount: options?.artistCount ?? 5,
+    albumCount: options?.albumCount ?? 5,
+    songCount: options?.songCount ?? 10,
+    ...libraryFilterParams(),
+  });
+  const r = data.searchResult3 ?? {};
+  return { artists: r.artist ?? [], albums: r.album ?? [], songs: r.song ?? [] };
+}
+
+/**
+ * Song-only paginated search3. Tolerates empty query — Navidrome returns all songs
+ * ordered by title in that case; strict Subsonic implementations may return nothing.
+ * Caller handles empty results gracefully (Tracks page falls back to its random pool).
+ */
+export async function searchSongsPaged(query: string, songCount: number, songOffset: number): Promise<SubsonicSong[]> {
+  const data = await api<{ searchResult3: { song?: SubsonicSong[] } }>('search3.view', {
+    query,
+    artistCount: 0,
+    albumCount: 0,
+    songCount,
+    songOffset,
+    ...libraryFilterParams(),
+  });
+  return data.searchResult3?.song ?? [];
+}