test(frontend): add specs for Tabs, AlertBanner, BackLink, useHashTab

4 new spec files adding 21 tests (7+6+4+4). Each component spec
includes brutalism invariant assertions (no rounded-md, no transition-*,
no shadow-*).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/tests/components/BackLink.test.tsx

@@ -0,0 +1,39 @@
+import { describe, it, expect } from 'vitest';
+import { render, screen } from '@testing-library/react';
+import { MemoryRouter } from 'react-router-dom';
+import { BackLink } from '@/components/BackLink';
+
+function renderBackLink(to: string, label: string) {
+  return render(
+    <MemoryRouter>
+      <BackLink to={to}>{label}</BackLink>
+    </MemoryRouter>,
+  );
+}
+
+describe('BackLink', () => {
+  it('renders children as link text', () => {
+    renderBackLink('/engagements', 'Back to engagements');
+    expect(screen.getByText('Back to engagements')).toBeInTheDocument();
+  });
+
+  it('renders as an anchor with the correct href', () => {
+    renderBackLink('/engagements', 'Back to engagements');
+    const link = screen.getByRole('link');
+    expect(link).toHaveAttribute('href', '/engagements');
+  });
+
+  it('has no rounded-md, transition-*, or shadow-* (brutalism)', () => {
+    renderBackLink('/engagements', 'Back');
+    const link = screen.getByRole('link');
+    expect(link).not.toHaveClass('rounded-md');
+    expect(link.className).not.toMatch(/transition-/);
+    expect(link.className).not.toMatch(/shadow-/);
+  });
+
+  it('contains the ArrowLeft icon (svg element)', () => {
+    const { container } = renderBackLink('/engagements', 'Back');
+    const svg = container.querySelector('svg');
+    expect(svg).toBeInTheDocument();
+  });
+});