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/locales/en/settings.ts

@@ -231,6 +231,10 @@ export const settings = {
   aboutContributorsLabel: 'Contributors',
   showChangelogOnUpdate: "Show 'What's New' on update",
   showChangelogOnUpdateDesc: "Show a discreet changelog banner above Now Playing after an update. Click opens the release notes; X dismisses it.",
+  libraryGridMaxColumnsTitle: 'Library card grids',
+  libraryGridMaxColumnsPerfHint: 'Higher values pack more tiles per row and can increase layout and painting work—noticeable on very large libraries or slower devices.',
+  libraryGridMaxColumnsRangeLabel: 'Maximum columns ({{min}}–{{max}})',
+  libraryGridMaxColumnsDesc: 'Applies to album, artist, playlist, radio, offline, and other card-based library views. Lower values use larger tiles and are usually easier on the CPU.',
   randomMixTitle: 'Random Mix Blacklist',
   luckyMixMenuTitle: 'Show Lucky Mix in menu',
   luckyMixMenuDesc: 'Enables Lucky Mix in Build a Mix and as a separate menu item when split navigation is on. Visible only when AudioMuse is enabled on the active server.',