Collapsible CodeBlock

[czarandy]

Feature

CodeBlock has no collapse/expand capability — the only length control is maxHeight, which scrolls (src/components/CodeBlock/CodeBlock.tsx:15-90). Long code samples in docs/chat contexts want to start collapsed to the header and expand on demand.

Add:

• isCollapsible (and defaultCollapsed, plus controlled isCollapsed/onCollapsedChange if cheap) props.
• A chevron toggle in the header, placed at the inline-start of the title so the title reads as the toggle's label; the whole header row can be the trigger.
• Smooth height animation that respects prefers-reduced-motion.
• Proper semantics: trigger is a real button with aria-expanded + aria-controls.

Notes

• Interaction with maxHeight should be defined (collapsed → header only; expanded → existing maxHeight behavior).
• Panda sva/cva recipe for the new slots per repo convention.
• Tests + Storybook example (collapsed-by-default story must not trap docs-site live demos in a surprising state — default stories should start expanded).