Foundations: design tokens, theming, dark mode & density #4

REQUIREMENTS §3, §9, §10.

• tokens.json (W3C DTCG) as the single source of truth → generated tokens.css
• --onu-primary drives every derived shade via OKLCH relative colour / color-mix(); auto contrast foreground (R9.1–R9.3)
• Granular tokens (colour, chart, sidebar, radius, fonts, density) exposed to Tailwind via @theme inline (R9.4–R9.5)
• Scoped themes; preset themes (Onu green, neutral, blue)
• Dark mode via .dark / [data-theme=dark]; ThemeProvider + useTheme (light/dark/system, persisted, no-flash script)
• Density modes compact | default | comfortable | touch (R3.2); elevation, spacing, radius, motion (tw-animate-css), reduced motion
• Prose styles for Latin + Thaana; print styles
• Design foundations page in examples (R3.10)