feat(settings): library card grid max columns in Appearance (4–12, default 6)

Persist libraryGridMaxColumns, wire useCardGridMetrics and card grid layout,
add i18n and settings search index; document performance hint in copy.

src/store/authStoreHelpers.ts

@@ -1,6 +1,9 @@
 import { clampStoredLoudnessPreAnalysisAttenuationRefDb } from '../utils/audio/loudnessPreAnalysisSlider';
 import {
   DEFAULT_LOUDNESS_PRE_ANALYSIS_ATTENUATION_DB,
+  DEFAULT_LIBRARY_GRID_MAX_COLUMNS,
+  LIBRARY_GRID_MAX_COLUMNS_MAX,
+  LIBRARY_GRID_MAX_COLUMNS_MIN,
   LOUDNESS_LUFS_PRESETS,
   MIX_MIN_RATING_FILTER_MAX_STARS,
   RANDOM_MIX_SIZE_OPTIONS,
@@ -41,6 +44,13 @@ export function clampRandomMixSize(v: number): number {
   return nearest;
 }
 
+/** Persisted max columns for library card grids (albums, artists, playlists, …). */
+export function clampLibraryGridMaxColumns(v: unknown): number {
+  const n = typeof v === 'number' ? v : Number(v);
+  if (!Number.isFinite(n)) return DEFAULT_LIBRARY_GRID_MAX_COLUMNS;
+  return Math.max(LIBRARY_GRID_MAX_COLUMNS_MIN, Math.min(LIBRARY_GRID_MAX_COLUMNS_MAX, Math.round(n)));
+}
+
 export function clampSkipStarThreshold(v: number): number {
   if (!Number.isFinite(v)) return 3;
   return Math.max(1, Math.min(99, Math.round(v)));