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/utils/cardGridLayout.test.ts

@@ -1,16 +1,26 @@
 import { describe, expect, it } from 'vitest';
-import { computeCardGridColumnCount, CARD_GRID_MAX_COLS } from './cardGridLayout';
+import { computeCardGridColumnCount } from './cardGridLayout';
+import { LIBRARY_GRID_MAX_COLUMNS_MAX, LIBRARY_GRID_MAX_COLUMNS_MIN } from '../store/authStoreDefaults';
 
 describe('computeCardGridColumnCount', () => {
-  it('never exceeds CARD_GRID_MAX_COLS', () => {
-    expect(computeCardGridColumnCount(20_000)).toBe(CARD_GRID_MAX_COLS);
+  it('never exceeds the configured max', () => {
+    expect(computeCardGridColumnCount(20_000, 6)).toBe(6);
+    expect(computeCardGridColumnCount(20_000, 4)).toBe(4);
+  });
+
+  it('clamps requested max to store-wide upper bound', () => {
+    expect(computeCardGridColumnCount(20_000, 99)).toBe(LIBRARY_GRID_MAX_COLUMNS_MAX);
+  });
+
+  it('clamps requested max to store-wide lower bound', () => {
+    expect(computeCardGridColumnCount(20_000, 2)).toBe(LIBRARY_GRID_MAX_COLUMNS_MIN);
   });
 
   it('returns at least one column', () => {
-    expect(computeCardGridColumnCount(50)).toBe(1);
+    expect(computeCardGridColumnCount(50, 6)).toBe(1);
   });
 
-  it('uses six columns on wide desktop widths', () => {
-    expect(computeCardGridColumnCount(1200)).toBe(6);
+  it('uses six columns on wide desktop when max allows', () => {
+    expect(computeCardGridColumnCount(1200, 6)).toBe(6);
   });
 });