refactor(settings): co-locate settings feature into features/settings

Theme infra (fixedThemes, theme utils), music-network presetIcon, audio-device
probe, library-index sync, and the auth*SettingsActions slices stay in the
core/global layer (consumed via @/ alias) to avoid inverting core->feature.

src/features/settings/components/PlaylistLayoutCustomizer.tsx

@@ -0,0 +1,44 @@
+import { useTranslation } from 'react-i18next';
+import { Download, FileUp, HardDrive, Lightbulb, Search } from 'lucide-react';
+import { usePlaylistLayoutStore, type PlaylistLayoutItemId } from '@/store/playlistLayoutStore';
+
+const PLAYLIST_LAYOUT_ICONS: Record<PlaylistLayoutItemId, typeof Search> = {
+  addSongs:     Search,
+  importCsv:    FileUp,
+  downloadZip:  Download,
+  offlineCache: HardDrive,
+  suggestions:  Lightbulb,
+};
+
+const PLAYLIST_LAYOUT_LABEL_KEYS: Record<PlaylistLayoutItemId, string> = {
+  addSongs:     'playlists.addSongs',
+  importCsv:    'playlists.importCSV',
+  downloadZip:  'playlists.downloadZip',
+  offlineCache: 'playlists.cacheOffline',
+  suggestions:  'playlists.suggestions',
+};
+
+export function PlaylistLayoutCustomizer() {
+  const { t } = useTranslation();
+  const items = usePlaylistLayoutStore(s => s.items);
+  const toggleItem = usePlaylistLayoutStore(s => s.toggleItem);
+
+  return (
+    <div style={{ padding: '4px 0' }}>
+      {items.map((it) => {
+        const Icon = PLAYLIST_LAYOUT_ICONS[it.id];
+        const label = t(PLAYLIST_LAYOUT_LABEL_KEYS[it.id]);
+        return (
+          <div key={it.id} className="sidebar-customizer-row">
+            <Icon size={16} style={{ color: 'var(--text-muted)', flexShrink: 0 }} />
+            <span style={{ flex: 1, fontSize: 14 }}>{label}</span>
+            <label className="toggle-switch" aria-label={label}>
+              <input type="checkbox" checked={it.visible} onChange={() => toggleItem(it.id)} />
+              <span className="toggle-track" />
+            </label>
+          </div>
+        );
+      })}
+    </div>
+  );
+}