feat: full dark/light theme — landing page + nav toggle button

- All 9 sections with light/dark M3 token pairs
- Theme toggle button (sun/moon) in navigation
- Auto-detect system preference + localStorage persistence
- Smooth theme transition animation

Files:
- platform/src/app/[locale]/page.tsx (dark: prefix for all colors)
- platform/src/components/Navigation.tsx (toggle button + dark: classes)

Author: AIwork4me