BorderRadiusControl: Improve element alignment

[mirka]

Follow-up issue after #35646

The Unlink button is currently aligned to flex-start, which is presumably to have it stay put when the corners are all unlinked. This was less noticeable in the previous design, but due to the taller input control it now looks more clearly misaligned in the linked state.

It would probably be better to:

• Center align the Unlink button with the first row of inputs
• Add a little more whitespace between the input and the slider

May need to coordinate with related issue #35602

Screenshots

Before (smaller input height)

After (taller input height)

[mirka]

Closing this for now since we won't be doing a blanket upsizing for the UnitControl component. May be resuscitated in the future.