Sync Figma tokens and add theme/mode switcher to Storybook

[sjoerdbeentjes]

What

• Adds scripts/sync-figma.ts — pulls Figma Variables API and writes DTCG JSON for all themes and modes. Run with pnpm sync:figma (needs FIGMA_TOKEN + FIGMA_FILE_ID in .env).
• Migrates Style Dictionary config to TypeScript (packages/tokens/scripts/style-dictionary.config.ts), executed via jiti.
• Style Dictionary now generates per-theme CSS diffs and a typed themes export in dist/index.js with full resolved token values per theme and mode.
• Adds theme and mode toolbar pickers to both Storybooks, driven by the keys of the generated themes object. Switching frameworks preserves the active theme and mode via the globals= URL param.

Token files

The DTCG JSON files in packages/tokens/src/ are generated — edit sync-figma.ts or re-run pnpm sync:figma rather than editing them by hand.

Checklist

• pnpm sync:figma writes JSON only, no CSS
• pnpm build generates CSS + JS from the JSON
• pnpm lint and pnpm format pass