accordion: Animate expanding and collapsing (#2672)

## Summary

The accordion content used to appear and disappear in one frame. Now its
height animates between 0 and the content height, and the chevron
rotates to follow, like the [shadcn
accordion](https://ui.shadcn.com/docs/components/base/accordion).

<img width="1039" height="974" alt="image"
src="https://github.com/user-attachments/assets/0f73e210-0563-46c9-9d4f-484023238f74"
/>

## Visual changes

- Chevron rotates 180° instead of swapping `ChevronUp` / `ChevronDown`.
- Bordered: no separator under the title when open.
- Not bordered: the separator moved from under the title to the bottom
of the item, so it sits below the content instead of between the title
and the content. The last item does not draw one.
- Larger vertical padding on the title, `font_medium` title, and muted
content text.

## Story

Wider section and longer content, so the height animation is visible.

## AI Assistance

Written with Claude.

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>