feat(ui): shared card grid with max six columns and virtualization

Add cardGridLayout helpers, useCardGridMetrics, useRemeasureGridVirtualizer, and VirtualCardGrid (TanStack row virtualization, always remeasure on layout changes).

Apply to Artists (grid + list unchanged policy), Albums, Composers grid, playlists, radio stations, offline library, and album-heavy browse/detail pages. Respect disableMainstageVirtualLists for a non-virtual grid with the same column rules.

Includes vitest coverage for column cap.

src/utils/cardGridLayout.test.ts

@@ -0,0 +1,16 @@
+import { describe, expect, it } from 'vitest';
+import { computeCardGridColumnCount, CARD_GRID_MAX_COLS } from './cardGridLayout';
+
+describe('computeCardGridColumnCount', () => {
+  it('never exceeds CARD_GRID_MAX_COLS', () => {
+    expect(computeCardGridColumnCount(20_000)).toBe(CARD_GRID_MAX_COLS);
+  });
+
+  it('returns at least one column', () => {
+    expect(computeCardGridColumnCount(50)).toBe(1);
+  });
+
+  it('uses six columns on wide desktop widths', () => {
+    expect(computeCardGridColumnCount(1200)).toBe(6);
+  });
+});