refactor(playlist): G.67 — extract usePlaylistCovers + usePlaylistSelection + usePlaylistSuggestions (cluster) (#634)

Three-cut cluster pulling tightly-scoped state + memo islands out of
PlaylistDetail.tsx as custom hooks. 659 → 565 LOC (−94). Each hook
returns the same names the page already used, so call sites stay
identical.

usePlaylistCovers(songs, customCoverId) — the 2×2 cover quad memo,
the four cover-quad URLs (with their stable cache keys to avoid the
buildCoverArtUrl-salt re-render loop documented in the comment),
the customCover fetch URL + cache key, and the blurred background
URL going through useCachedUrl. Returns coverQuadUrls /
customCoverFetchUrl / customCoverCacheKey / resolvedBgUrl. The page
no longer imports buildCoverArtUrl / coverArtCacheKey / useCachedUrl
directly.

usePlaylistSelection(songs, setSongs, savePlaylist) — selectedIds /
lastSelectedIdx state + toggleSelect (with shift-range support) +
allSelected + toggleAll + bulkRemove (savePlaylist + setSongs +
clears selection). Returns the same shape the tracklist props
already destructured. savePlaylist moves a few lines up in
PlaylistDetail so the hook can be called with it.

usePlaylistSuggestions(songs, playlist.id) — suggestions state +
loadSuggestions (genre-weighted random pull, top 10) + the auto-load
useEffect that fires on playlist change. Returns setSuggestions too
so addSong on the page can still filter the just-added song out of
the suggestions strip. The page drops the getRandomSongs import.

Pure code move otherwise — no behaviour change.

src/hooks/usePlaylistCovers.ts

@@ -0,0 +1,54 @@
+import { useMemo } from 'react';
+import { buildCoverArtUrl, coverArtCacheKey } from '../api/subsonicStreamUrl';
+import type { SubsonicSong } from '../api/subsonicTypes';
+import { useCachedUrl } from '../components/CachedImage';
+
+export interface PlaylistCovers {
+  coverQuadUrls: ({ src: string; cacheKey: string } | null)[];
+  customCoverFetchUrl: string | null;
+  customCoverCacheKey: string | null;
+  resolvedBgUrl: string | null;
+}
+
+export function usePlaylistCovers(songs: SubsonicSong[], customCoverId: string | null): PlaylistCovers {
+  const coverQuad = useMemo(() => {
+    const seen = new Set<string>();
+    const result: string[] = [];
+    for (const s of songs) {
+      if (s.coverArt && !seen.has(s.coverArt)) {
+        seen.add(s.coverArt);
+        result.push(s.coverArt);
+        if (result.length === 4) break;
+      }
+    }
+    return result;
+  }, [songs]);
+
+  // Stable fetch URLs + cache keys for the 2×2 grid and blurred background.
+  // buildCoverArtUrl generates a new crypto salt on every call, so these MUST
+  // be memoized — otherwise every render produces new URLs, useCachedUrl
+  // re-triggers, state updates, another render → infinite flicker loop.
+  const coverQuadUrls = useMemo(() =>
+    Array.from({ length: 4 }, (_, i) => {
+      const coverId = coverQuad[i % Math.max(1, coverQuad.length)];
+      if (!coverId) return null;
+      return { src: buildCoverArtUrl(coverId, 200), cacheKey: coverArtCacheKey(coverId, 200) };
+    }),
+  [coverQuad]);
+
+  const effectiveBgId = customCoverId ?? coverQuad[0] ?? '';
+  const bgFetchUrl = useMemo(() => buildCoverArtUrl(effectiveBgId, 300), [effectiveBgId]);
+  const bgCacheKey = useMemo(() => coverArtCacheKey(effectiveBgId, 300), [effectiveBgId]);
+  const resolvedBgUrl = useCachedUrl(bgFetchUrl, bgCacheKey);
+
+  const customCoverFetchUrl = useMemo(
+    () => customCoverId ? buildCoverArtUrl(customCoverId, 300) : null,
+    [customCoverId],
+  );
+  const customCoverCacheKey = useMemo(
+    () => customCoverId ? coverArtCacheKey(customCoverId, 300) : null,
+    [customCoverId],
+  );
+
+  return { coverQuadUrls, customCoverFetchUrl, customCoverCacheKey, resolvedBgUrl };
+}