refactor(playlists): G.79 — extract smart helpers + cover images + 2 lazy-fetch hooks (cluster) (#646)

Four-cut cluster opening the Playlists refactor. 1039 → 763 LOC
(−276).

playlistsSmart — full smart-playlist module: SMART_PREFIX /
LIMIT_MAX / YEAR_MIN / YEAR_MAX constants, GenreMode / YearMode /
SmartFilters / PendingSmartPlaylist / NdSmartRuleNode types,
defaultSmartFilters seed, clampYear / isSmartPlaylistName /
displayPlaylistName / asRecord helpers, parseSmartRulesToFilters
(the Navidrome JSON rule walker), and buildSmartRulesPayload (the
reverse — page-state → Navidrome JSON). The payload builder
becomes parameterized on the filters object so it lives outside
the component.

PlaylistCoverImages — two tiny CachedImage wrappers
(PlaylistSmartCoverCell for the 200 px collage cells,
PlaylistCardMainCover for the 256 px main card cover).

useSmartCoverCollage — replaces the inline useEffect that builds
the 2×2 cover collage for each smart playlist (pulls playlist
tracks, filters to active library scope, collects up to four
unique cover-art ids). Returns the per-playlist id map and
re-fetches on playlist list change or library filter version bump.

usePlaylistsLibraryScopeCounts — replaces the inline useEffect
that recomputes per-playlist song count + total duration under
the current library scope. Chunked into batches of four parallel
fetches.

Playlists drops the inline definitions; pure code move otherwise.

src/hooks/useSmartCoverCollage.ts

@@ -0,0 +1,57 @@
+import { useEffect, useState } from 'react';
+import { filterSongsToActiveLibrary } from '../api/subsonicLibrary';
+import { getPlaylist } from '../api/subsonicPlaylists';
+import type { SubsonicPlaylist } from '../api/subsonicTypes';
+import { isSmartPlaylistName } from '../utils/playlistsSmart';
+
+/**
+ * Build the 2×2 cover collage for each smart playlist. Pulls each smart
+ * playlist's tracks (filtered to the active library scope) and collects up
+ * to four unique cover-art IDs. Re-runs when the playlist list changes or
+ * when the active library filter version bumps.
+ */
+export function useSmartCoverCollage(
+  playlists: SubsonicPlaylist[],
+  musicLibraryFilterVersion: number,
+): Record<string, string[]> {
+  const [smartCoverIdsByPlaylist, setSmartCoverIdsByPlaylist] = useState<Record<string, string[]>>({});
+
+  useEffect(() => {
+    let cancelled = false;
+    const run = async () => {
+      const smart = playlists.filter(pl => isSmartPlaylistName(pl.name));
+      if (smart.length === 0) {
+        if (!cancelled) setSmartCoverIdsByPlaylist({});
+        return;
+      }
+      const rows = await Promise.all(
+        smart.map(async (pl) => {
+          try {
+            const { songs } = await getPlaylist(pl.id);
+            const filtered = await filterSongsToActiveLibrary(songs);
+            const ids: string[] = [];
+            const seen = new Set<string>();
+            for (const s of filtered) {
+              const cid = s.coverArt;
+              if (!cid || seen.has(cid)) continue;
+              seen.add(cid);
+              ids.push(cid);
+              if (ids.length >= 4) break;
+            }
+            return [pl.id, ids] as const;
+          } catch {
+            return [pl.id, [] as string[]] as const;
+          }
+        }),
+      );
+      if (cancelled) return;
+      const next: Record<string, string[]> = {};
+      for (const [id, ids] of rows) next[id] = ids;
+      setSmartCoverIdsByPlaylist(next);
+    };
+    run();
+    return () => { cancelled = true; };
+  }, [playlists, musicLibraryFilterVersion]);
+
+  return smartCoverIdsByPlaylist;
+}