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/package.json

@@ -25,6 +25,7 @@
 		"@tailwindcss/vite": "^4.2.2",
 		"@testing-library/jest-dom": "^6.9.1",
 		"@testing-library/svelte": "^5.3.1",
+		"@types/node": "^25.5.0",
 		"@vitest/ui": "^4.1.2",
 		"jsdom": "^29.0.1",
 		"openapi-typescript": "^7.13.0",