fix(chat): nest the reply quote concentrically in its bubble

[bmc08gt]

The citation panel inside a reply drew an 8dp corner inside the bubble's 12, with a 16dp gap down its sides, 10 at the top and 6 under it. Four numbers, none derived from the others, so neither arc ran parallel to the other and there were three different gaps to read the panel against.

The surround

BubbleDefaults names the bubble's own geometry, and surroundInset is one gap used on the panel's top, leading and trailing edges and again between the panel and the body beneath it. The corner then has a single surround to be concentric with: the bubble's radius less that gap.

	before	after
panel inset, top	10dp	10dp
panel inset, leading/trailing	16dp	10dp
gap panel to body	6dp	10dp
panel corner	8dp	4dp (12 − 10, floored)
panel gutters	8dp / 6dp	10dp / 5dp

The bubble hands a reply the surround as its horizontal padding and the body puts the difference back, which is how the panel reaches past the text on both sides. A bubble with no quote passes the bubble's own padding, so the difference is zero and nothing about it moves.

Matching the widths

The panel stretches to the bubble's width when it is the narrower of the two. A Column left a short quote hanging inside a wider reply, and the panel is a filled surface, so the slack read as a notch cut out of the bubble.

fillMaxWidth is no help: it fills the incoming maximum, not the sibling's width, so every reply would square off against the widest bubble the transcript allows. QuotedBody measures the body first and makes its width the panel's floor instead. A quote longer than the reply keeps its own width and carries it out to the bubble, which is what makes this a floor rather than a fixed width. A bubble with no quote never reaches the layout at all, so short messages keep hugging their text.

Tokens

Every measurement that has a token now reads one. CodeTheme.shapes.medium and .tiny supply the bubble's two radii through a new CornerBasedShape.cornerRadius(), and the paddings come off CodeTheme.dimens.staticGrid.

That grid is 5pt and has no 16dp step, so the bubble's horizontal padding moves from 16dp to 15dp — a 1dp change to every bubble in the transcript, not just replies. Say the word and I will keep 16dp as an off-grid literal instead.

The panel's accent width, flag size and alphas stay as literals. They came from iOS and have no token to come from.

Against iOS

iOS landed the same surround in code-payments/code-ios-app#735. One difference here is deliberate: the radius is floored at the bubble's flattened corner. iOS takes 12 − 9 = 3 straight, but that is a .continuous curve; the same number as Compose's circular arc draws a hard rectangle inside a rounded one.

The other two commits squashed into that PR fix a recycled bubble keeping a stale quote width and a 42pt width floor on quote-less bubbles. Both are UIKit constraint and reuse bugs with no equivalent here, since the panel is not composed at all when there is no quote.