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__/signal.spec.tsx

@@ -0,0 +1,45 @@
+import { render, screen } from '@testing-library/react';
+import { describe, expect, it } from 'vitest';
+import { Signal } from '../signal';
+
+describe('Signal', () => {
+  it('renders without a label', () => {
+    const { container } = render(<Signal direction="up" />);
+    expect(container.querySelector('span')).toBeInTheDocument();
+  });
+
+  it('renders label text when provided', () => {
+    render(<Signal direction="up" label="Tăng" />);
+    expect(screen.getByText('Tăng')).toBeInTheDocument();
+  });
+
+  it('renders icon with aria-hidden for up direction', () => {
+    const { container } = render(<Signal direction="up" label="Up" />);
+    expect(container.querySelector('[aria-hidden="true"]')).toBeInTheDocument();
+  });
+
+  it('renders icon with aria-hidden for down direction', () => {
+    const { container } = render(<Signal direction="down" label="Down" />);
+    expect(container.querySelector('[aria-hidden="true"]')).toBeInTheDocument();
+  });
+
+  it('renders icon with aria-hidden for neutral direction', () => {
+    const { container } = render(<Signal direction="neutral" label="Neutral" />);
+    expect(container.querySelector('[aria-hidden="true"]')).toBeInTheDocument();
+  });
+
+  it('renders as an inline span', () => {
+    const { container } = render(<Signal direction="up" label="x" />);
+    expect(container.firstChild?.nodeName).toBe('SPAN');
+  });
+
+  it('merges custom className onto root span', () => {
+    const { container } = render(<Signal direction="up" className="extra" label="x" />);
+    expect(container.firstChild).toHaveClass('extra');
+  });
+
+  it('contains an svg icon', () => {
+    const { container } = render(<Signal direction="up" />);
+    expect(container.querySelector('svg')).toBeInTheDocument();
+  });
+});