feat: implement modern UI/UX design system (#409)

- Add comprehensive design token system (colors, typography, spacing)
- Create 12 new UI components with Radix UI primitives
- Add layout components (PageShell, StatsCard, EmptyState, DataTable)
- Polish all pages with new component library
- Improve accessibility with WCAG 2.1 compliance
- Add dark mode support with semantic color tokens
- Update 947 tests to match new UI patterns

Closes #409

frontend/src/components/__tests__/CertificateStatusCard.test.tsx

@@ -131,7 +131,7 @@ describe('CertificateStatusCard', () => {
     renderWithRouter(<CertificateStatusCard certificates={[]} hosts={[]} />)
 
     expect(screen.getByText('0')).toBeInTheDocument()
-    expect(screen.getByText('0 valid')).toBeInTheDocument()
+    expect(screen.getByText('No certificates')).toBeInTheDocument()
   })
 })