refactor(utils): group utils/ files into topic folders (Phase L, part 1) (#689)

111 of 122 top-level src/utils/ files move into 16 topic folders (audio,
cache, cover, share, server, playback, playlist, deviceSync, waveform,
mix, format, export, changelog, ui, perf, componentHelpers). True
singletons with no cluster stay at the utils/ root.

Pure file-move: a path-aware codemod rewrote 539 relative-import
specifiers across 275 files; no logic touched. The hot-path coverage
gate list (.github/frontend-hot-path-files.txt) is updated to the new
paths for the 11 gated utils files — a mechanical consequence of the
move, not a CI change. tsc is green.

src/utils/playback/buildInfiniteQueueCandidates.ts

@@ -0,0 +1,61 @@
+import { getSimilarSongs2, getTopSongs } from '../../api/subsonicArtists';
+import { getRandomSongs } from '../../api/subsonicLibrary';
+import type { Track } from '../../store/playerStoreTypes';
+import {
+  enrichSongsForMixRatingFilter,
+  getMixMinRatingsConfigFromAuth,
+  passesMixMinRatings,
+} from '../mix/mixRatingFilter';
+import { shuffleArray } from './shuffleArray';
+import { songToTrack } from './songToTrack';
+/**
+ * Infinite queue source strategy (Instant Mix-like):
+ * 1) Prefer artist-driven candidates (Top + Similar) around the current track.
+ * 2) Fallback to random songs when artist-driven fetches are empty.
+ */
+export async function buildInfiniteQueueCandidates(
+  seedTrack: Track | null,
+  existingIds: Set<string>,
+  count = 5,
+): Promise<Track[]> {
+  const RANDOM_TOPUP_BATCH_SIZE = Math.max(10, count * 2);
+  const RANDOM_TOPUP_MAX_BATCHES = 8;
+  const artistId = seedTrack?.artistId?.trim() || null;
+  const artistName = seedTrack?.artist?.trim() || null;
+
+  const [similar, top] = await Promise.all([
+    artistId ? getSimilarSongs2(artistId).catch(() => []) : Promise.resolve([]),
+    artistName ? getTopSongs(artistName).catch(() => []) : Promise.resolve([]),
+  ]);
+
+  const seedId = seedTrack?.id ?? null;
+  const mixCfg = getMixMinRatingsConfigFromAuth();
+  const mixedSources = [...top, ...similar];
+  const filteredMixedSongs = mixCfg.enabled
+    ? (await enrichSongsForMixRatingFilter(mixedSources, mixCfg)).filter(s => passesMixMinRatings(s, mixCfg))
+    : mixedSources;
+  const out: Track[] = shuffleArray(
+    filteredMixedSongs
+      .map(songToTrack)
+      .filter(t => t.id !== seedId && !existingIds.has(t.id)),
+  )
+    .slice(0, count)
+    .map(t => ({ ...t, autoAdded: true as const }));
+
+  const seenIds = new Set<string>([...existingIds, ...out.map(t => t.id)]);
+  for (let b = 0; out.length < count && b < RANDOM_TOPUP_MAX_BATCHES; b++) {
+    const random = await getRandomSongs(RANDOM_TOPUP_BATCH_SIZE, seedTrack?.genre).catch(() => []);
+    if (!random.length) break;
+    const filteredRandomSongs = mixCfg.enabled
+      ? (await enrichSongsForMixRatingFilter(random, mixCfg)).filter(s => passesMixMinRatings(s, mixCfg))
+      : random;
+    for (const track of shuffleArray(filteredRandomSongs.map(songToTrack))) {
+      if (track.id === seedId || seenIds.has(track.id)) continue;
+      out.push({ ...track, autoAdded: true as const });
+      seenIds.add(track.id);
+      if (out.length >= count) break;
+    }
+  }
+
+  return out.slice(0, count);
+}