style(joint-react): CSS design token overhaul — rename layer, expand palette, and update examples

[Geliogabalus]

Summary

• Renames the CSS layer from jointjs to joint for consistency
• Expands the color palette with a full dark/light scale and semantic tokens (destructive, success, invalid)
• Adds spacing (--space-*) and border-radius (--radius-*) design tokens
• Re-maps existing --jj-color-* variables to use the new palette tokens
• Removes hard-coded inline styles from several Storybook examples in favour of CSS-driven theming

Test plan

• Verify Storybook stories render correctly with the new CSS layer name
• Check that CSS custom properties resolve correctly (no broken references)
• Confirm removed inline styles in examples fall back to CSS defaults as expected
• Smoke-test with-css-theme, with-default-element, and with-graph-neighbors stories

🤖 Generated with Claude Code