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/resolvePlaybackUrl.ts

@@ -0,0 +1,63 @@
+import { buildStreamUrl } from '../../api/subsonicStreamUrl';
+import { useOfflineStore } from '../../store/offlineStore';
+import { useHotCacheStore } from '../../store/hotCacheStore';
+
+/** Same resolution order as {@link resolvePlaybackUrl} — for UI hints only. */
+export type PlaybackSourceKind = 'offline' | 'hot' | 'stream';
+
+/**
+ * Subsonic `buildStreamUrl()` rotates `t`/`s` on every call; Rust matches by `id` (see `playback_identity`).
+ */
+export function streamUrlTrackId(url: string): string | null {
+  if (!url.includes('stream.view')) return null;
+  try {
+    const fromUrl = new URL(url).searchParams.get('id');
+    if (fromUrl) return fromUrl;
+  } catch {
+    // Fallback for non-standard/relative URLs: parse query manually.
+  }
+  const q = url.split('?')[1];
+  if (!q) return null;
+  for (const part of q.split('&')) {
+    const [k, v = ''] = part.split('=');
+    if (k === 'id') {
+      try {
+        return decodeURIComponent(v);
+      } catch {
+        return v;
+      }
+    }
+  }
+  return null;
+}
+
+/**
+ * @param enginePreloadedTrackId — song id for which `audio_preload` finished into the engine RAM slot
+ *   (parsed from `audio:preload-ready` payload URL).
+ */
+export function getPlaybackSourceKind(
+  trackId: string,
+  serverId: string,
+  enginePreloadedTrackId: string | null = null,
+): PlaybackSourceKind {
+  if (useOfflineStore.getState().getLocalUrl(trackId, serverId)) return 'offline';
+  if (useHotCacheStore.getState().getLocalUrl(trackId, serverId)) return 'hot';
+  const resolved = resolvePlaybackUrl(trackId, serverId);
+  if (
+    !resolved.startsWith('psysonic-local://')
+    && enginePreloadedTrackId
+    && trackId === enginePreloadedTrackId
+  ) {
+    return 'hot';
+  }
+  return 'stream';
+}
+
+/** Offline library → hot playback cache → HTTP stream. */
+export function resolvePlaybackUrl(trackId: string, serverId: string): string {
+  const offline = useOfflineStore.getState().getLocalUrl(trackId, serverId);
+  if (offline) return offline;
+  const hot = useHotCacheStore.getState().getLocalUrl(trackId, serverId);
+  if (hot) return hot;
+  return buildStreamUrl(trackId);
+}