refactor(block): swap animation with transition

[Elijbet]

Related Issue: #12669

Summary

As part of aligning animation implementation on expanded/collapsed components, swap animation with transition on block.

[Elijbet]

Why transition is better for expanded/collapsed components:

1. Automatic reversal - When you remove expanded, it automatically fades out. With animation, you'd need a separate fade-out animation.
2. Simpler code - No keyframes needed for a simple 0→1 change.
3. Better for state-driven UIs - React/Vue/Lit etc. work naturally with class/state toggles.
4. Slightly better performance - Transitions are optimized for property changes.

Animation with keyframes approach works fine, but it's typically used when you need that animation to play once on render, or when the animation has multiple steps that transition can't handle elegantly.