refactor(artists): I.4 — split Artists.tsx 520 → 233 LOC across 6 files (#676)

* refactor(artists): extract helpers + constants

Pull ALL_SENTINEL / ALPHABET / ARTIST_LIST_* row-height estimates,
the ArtistListFlatRow union, CTP_COLORS palette, and the deterministic
nameColor / nameInitial helpers into utils/artistsHelpers.ts.

Artists.tsx: 520 → 496 LOC.

* refactor(artists): extract ArtistAvatars subcomponents

Pull ArtistCardAvatar (300px, grid view) and ArtistRowAvatar (64px, list
view) into components/artists/ArtistAvatars.tsx. Both fall back to a
hashed-Catppuccin monogram when artist images are off or no cover art
is available.

Artists.tsx: 496 → 436 LOC.

* refactor(artists): extract useArtistsFiltering hook

Bundle the letter/text/star filter pipeline, visible-slice memo,
group-by-letter, and the virtualizer flat-rows list into
hooks/useArtistsFiltering.ts. List-view-only outputs short-circuit when
grid view is active.

Artists.tsx: 436 → 386 LOC.

* refactor(artists): extract useArtistsInfiniteScroll hook

Bundle visibleCount + loadingMore state, the sentinel
IntersectionObserver, loadMore callback, and the filter-change reset
into hooks/useArtistsInfiniteScroll.ts. The observer no longer takes
hasMore — the sentinel element only mounts while there is more data,
so the observer attaches/detaches naturally with it.

Artists.tsx: 386 → 370 LOC.

* refactor(artists): extract ArtistsGridView + ArtistsListView

Move the grid card layout to components/artists/ArtistsGridView.tsx and
the dual-path list layout (non-virtualized fallback + virtualized stream)
to components/artists/ArtistsListView.tsx. Both paths now share an
internal ArtistListRow component so click + context-menu behaviour is
identical regardless of which renderer is active.

Artists.tsx: 370 → 233 LOC.

src/hooks/useArtistsInfiniteScroll.ts

@@ -0,0 +1,61 @@
+import React, { useCallback, useEffect, useRef, useState } from 'react';
+
+interface UseArtistsInfiniteScrollArgs {
+  pageSize: number;
+  resetDeps: ReadonlyArray<unknown>;
+}
+
+interface UseArtistsInfiniteScrollResult {
+  visibleCount: number;
+  loadingMore: boolean;
+  observerTarget: React.RefObject<HTMLDivElement | null>;
+  loadMore: () => void;
+}
+
+/**
+ * Page through the artists list with a sentinel-driven
+ * IntersectionObserver. `pageSize` is dynamic because artist-images
+ * mode wants smaller batches to keep disk I/O sane on big libraries
+ * (5000+ artists).
+ *
+ * `resetDeps` is the list of values that should snap `visibleCount`
+ * back to one page — filter text, letter pick, starred-only,
+ * view-mode, page-size itself.
+ *
+ * The observer doesn't take a `hasMore` flag — the page only renders
+ * the sentinel `<div ref={observerTarget}>` while there is more data,
+ * so the observer naturally disconnects when the last page is reached
+ * (the cleanup runs as the sentinel unmounts).
+ */
+export function useArtistsInfiniteScroll({
+  pageSize,
+  resetDeps,
+}: UseArtistsInfiniteScrollArgs): UseArtistsInfiniteScrollResult {
+  const [visibleCount, setVisibleCount] = useState(pageSize);
+  const [loadingMore, setLoadingMore] = useState(false);
+  const observerTarget = useRef<HTMLDivElement>(null);
+
+  const loadMore = useCallback(() => {
+    if (loadingMore) return;
+    setLoadingMore(true);
+    setVisibleCount(prev => prev + pageSize);
+    setTimeout(() => setLoadingMore(false), 100);
+  }, [loadingMore, pageSize]);
+
+  useEffect(() => {
+    setVisibleCount(pageSize);
+    // resetDeps is intentionally spread into the dep array.
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [pageSize, ...resetDeps]);
+
+  useEffect(() => {
+    const observer = new IntersectionObserver(
+      entries => { if (entries[0].isIntersecting) loadMore(); },
+      { rootMargin: '200px' },
+    );
+    if (observerTarget.current) observer.observe(observerTarget.current);
+    return () => observer.disconnect();
+  }, [loadMore]);
+
+  return { visibleCount, loadingMore, observerTarget, loadMore };
+}