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/subsonicStarRating.ts

@@ -0,0 +1,64 @@
+import { api, libraryFilterParams } from './subsonicClient';
+import type {
+  EntityRatingSupportLevel,
+  StarredResults,
+  SubsonicAlbum,
+  SubsonicArtist,
+  SubsonicSong,
+} from './subsonicTypes';
+
+export async function getStarred(): Promise<StarredResults> {
+  const data = await api<{
+    starred2: {
+      artist?: SubsonicArtist[];
+      album?: SubsonicAlbum[];
+      song?: SubsonicSong[];
+    }
+  }>('getStarred2.view', { ...libraryFilterParams() });
+  const r = data.starred2 ?? {};
+  return { artists: r.artist ?? [], albums: r.album ?? [], songs: r.song ?? [] };
+}
+
+export async function star(id: string, type: 'song' | 'album' | 'artist' = 'album'): Promise<void> {
+  const params: Record<string, string> = {};
+  if (type === 'song') params.id = id;
+  if (type === 'album') params.albumId = id;
+  if (type === 'artist') params.artistId = id;
+  await api('star.view', params);
+}
+
+export async function unstar(id: string, type: 'song' | 'album' | 'artist' = 'album'): Promise<void> {
+  const params: Record<string, string> = {};
+  if (type === 'song') params.id = id;
+  if (type === 'album') params.albumId = id;
+  if (type === 'artist') params.artistId = id;
+  await api('unstar.view', params);
+}
+
+export async function setRating(id: string, rating: number): Promise<void> {
+  await api('setRating.view', { id, rating });
+  // Cached song lists keyed by rating (e.g. Tracks → Highly Rated rail) become
+  // stale immediately. Lazy-import to keep the module dep direction
+  // subsonic ← navidromeBrowse and avoid pulling Tauri internals into shared
+  // type-only consumers.
+  void import('./navidromeBrowse').then(m => m.ndInvalidateSongsCache()).catch(() => {});
+}
+
+/**
+ * Probe server for OpenSubsonic extensions. When `openSubsonic: true`, we treat album/artist
+ * rating as supported (same `setRating.view` + entity id); otherwise track-only.
+ */
+export async function probeEntityRatingSupport(): Promise<EntityRatingSupportLevel> {
+  try {
+    const data = await api<{ openSubsonic?: boolean; openSubsonicExtensions?: unknown[] }>(
+      'getOpenSubsonicExtensions.view',
+      {},
+      8000,
+    );
+    if (data.openSubsonic === true) return 'full';
+    if (Array.isArray(data.openSubsonicExtensions)) return 'full';
+    return 'track_only';
+  } catch {
+    return 'track_only';
+  }
+}