feat: introduce design token system foundation

- Add semantic OKLch color tokens with light/dark themes
- Add typography, spacing, effects, breakpoints, z-index tokens
- Refactor base.css with backward-compatible aliases
- Add prefers-reduced-motion support
- Add MIGRATION.md for Phase 2 component audit

static/css/tokens/index.css

@@ -0,0 +1,6 @@
+@import 'colors.css';
+@import 'typography.css';
+@import 'spacing.css';
+@import 'effects.css';
+@import 'breakpoints.css';
+@import 'z-index.css';