feat(design-system): add Tailwind 4 @theme tokens, fonts, and completeness tests

- Load Fraunces, DM Sans, DM Mono via Google Fonts preconnect in app.html
- Define all design tokens in @theme block: neutrals, green/yellow/blue/
  purple/orange scales, spacing (--space-1..20), radii, shadows, button base
- Note --green-dark as button background (--green fails WCAG AA with white)
- Add @types/node for Node fs/path usage in design-system tests

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

frontend/src/lib/design-system/tokens.test.ts

@@ -0,0 +1,57 @@
+import { describe, it, expect } from 'vitest';
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+
+const css = readFileSync(resolve(__dirname, '../../app.css'), 'utf-8');
+
+const requiredTokens = [
+	// Fonts
+	'--font-display',
+	'--font-sans',
+	'--font-mono',
+	// Neutrals
+	'--color-page',
+	'--color-surface',
+	'--color-subtle',
+	'--color-border',
+	'--color-text',
+	'--color-text-muted',
+	// Green scale
+	'--green-tint',
+	'--green-light',
+	'--green',
+	'--green-dark',
+	'--green-deeper',
+	// Yellow scale
+	'--yellow-tint',
+	'--yellow-light',
+	'--yellow',
+	'--yellow-dark',
+	'--yellow-text',
+	// Status
+	'--color-error',
+	// Spacing
+	'--space-1',
+	'--space-4',
+	'--space-8',
+	'--space-12',
+	'--space-16',
+	'--space-20',
+	// Radii
+	'--radius-xs',
+	'--radius-sm',
+	'--radius-md',
+	'--radius-lg',
+	'--radius-xl',
+	'--radius-full',
+	// Shadows
+	'--shadow-card',
+	'--shadow-raised',
+	'--shadow-overlay'
+];
+
+describe('design token completeness', () => {
+	it.each(requiredTokens)('%s is defined in app.css', (token) => {
+		expect(css).toContain(token);
+	});
+});