test(web): add Vitest+RTL tests for 15 design-system presentational components

Covers Badge, Divider, EmptyState, Numeric, PriceDelta, Signal, Skeleton,
StatusChip, Surface, StatCard, KpiCard, DensityToggle, Footer, MarketIndex,
CompactHeader — rendering, variants, props, a11y attributes, className merging.

All 1139 web tests pass. Zustand persist store mocked for DensityToggle to
avoid jsdom localStorage incompatibility.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/components/design-system/__tests__/empty-state.spec.tsx

@@ -0,0 +1,40 @@
+import { render, screen } from '@testing-library/react';
+import { describe, expect, it } from 'vitest';
+import { EmptyState } from '../empty-state';
+
+describe('EmptyState', () => {
+  it('renders title', () => {
+    render(<EmptyState title="Không có dữ liệu" />);
+    expect(screen.getByText('Không có dữ liệu')).toBeInTheDocument();
+  });
+
+  it('renders description when provided', () => {
+    render(<EmptyState title="Title" description="Mô tả phụ" />);
+    expect(screen.getByText('Mô tả phụ')).toBeInTheDocument();
+  });
+
+  it('does not render description when omitted', () => {
+    render(<EmptyState title="Title" />);
+    expect(screen.queryByText('Mô tả phụ')).not.toBeInTheDocument();
+  });
+
+  it('renders icon node', () => {
+    render(<EmptyState title="Title" icon={<span data-testid="icon" />} />);
+    expect(screen.getByTestId('icon')).toBeInTheDocument();
+  });
+
+  it('renders action node', () => {
+    render(
+      <EmptyState
+        title="Title"
+        action={<button data-testid="action">Thêm mới</button>}
+      />,
+    );
+    expect(screen.getByTestId('action')).toBeInTheDocument();
+  });
+
+  it('merges custom className', () => {
+    render(<EmptyState data-testid="es" title="T" className="custom" />);
+    expect(screen.getByTestId('es')).toHaveClass('custom');
+  });
+});