ui: fix system message row stretching to full prompt height in WebKit
The expand button wrapping the system message card defaulted to
inline-block. WebKit takes an inline-block's baseline from its last
line even when that line is clipped by the card's max-height, so the
row grew to the height of the whole prompt and left a large empty gap
below the card. Make the button a block so the row matches the card.
Assisted-by: Claude (Opus 5.5 Medium)