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/pages/__tests__/SMTPSettings.test.tsx

@@ -33,8 +33,8 @@ describe('SMTPSettings', () => {
 
     renderWithQueryClient(<SMTPSettings />)
 
-    // Should show loading spinner
-    expect(document.querySelector('.animate-spin')).toBeTruthy()
+    // Should show loading skeletons (Skeleton components don't use animate-spin)
+    expect(document.querySelectorAll('[class*="animate-pulse"]').length).toBeGreaterThan(0)
   })
 
   it('renders SMTP form with existing config', async () => {