feat: design token governance

[wangdicoder]

Summary

• Design token governance: Added JSON source-of-truth token registry in packages/tokens/source/ with component-level and semantic token definitions, dark theme overrides, schema validation, and a v2 build pipeline
• SCSS token migration: Migrated all 60+ component stylesheets in packages/react/ to reference design token CSS custom properties instead of hardcoded values
• CSS Grid layout system: New Grid and GridItem components with named areas, responsive breakpoints, auto-fit, and alignment support
• Theme Studio rebuild: Replaced the old theme editor with a new Theme Studio featuring runtime presets, tweakcn integration, theme document persistence, and live preview
• Charts package: Added tests, build scripts, and style improvements for @tiny-design/charts

Release

• Bump type: minor (new features, backwards compatible)
• Affected packages: @tiny-design/react, @tiny-design/tokens (fixed version group — @tiny-design/icons will also bump)

Test plan

• Run pnpm build — all packages compile successfully
• Run pnpm test — all existing and new tests pass
• Run pnpm dev — docs site loads, Theme Studio works, Grid demos render correctly
• Verify component styles still look correct after SCSS token migration
• Verify dark theme works with new token overrides

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