Theming v2: design tokens via CSS custom properties + JSON schema for theme files

[PAMulligan]

Summary

Upgrade the existing accent-color override to a full design-token system: a JSON theme file (validated against a JSON schema) that drives every color, radius, shadow, and font in the widget via CSS custom properties.

Motivation

Clients want to match their brand precisely, not just pick an accent. A schema-validated theme file also enables the playground site to act as a visual theme editor.

Acceptance Criteria

• All visual decisions expressed as CSS custom properties (--cl-color-accent, --cl-color-surface, --cl-radius-md, --cl-shadow-elevated, etc.)
• JSON schema published at https://claudius.dev/schema/theme.v1.json
• theme: ClaudiusTheme | string prop accepts either an object or a URL to a theme JSON file
• Built-in themes: default, minimal, playful, corporate
• Playground site gains a theme editor that exports the JSON file
• Documented migration path from the v1 accentColor-only API