feat: improve docs navigation and Pro CTAs

[starc007]

Summary

• make component documentation full width with a fixed hierarchical page navigation
• add formal beUI brand tokens and typography updates
• add the beUI Pro sidebar card and shared animated rainbow CTA
• promote Explore Pro as the primary hero action while keeping Browse components secondary
• prevent route and smooth-scroll navigation state shifts

Validation

• bun run check
• git diff --check
• browser-tested component navigation, route transitions, Pro card, and hero CTA layout