fix(chat): stop the send bounce and drop the receipt reveal's scale

[bmc08gt]

Two animation bugs when sending a message, both reported against the iOS behaviour.

The bubble bounced on send

MessageList ran animateScrollToItem(0) for the newly sent message while the previous message's receipt label collapsed its height away above it. An animated scroll resolves its target offset when it starts, so the collapse moved that target mid-flight and the list overshot and corrected. That correction is what read as the bubble bouncing.

Own messages now use requestScrollToItem(0, 0). The new bubble is anchored during the next measure pass, in the same frame the receipt collapses, so there is no target to move. Incoming messages keep the animated scroll when the reader is near the bottom.

iOS has no scroll here to race in the first place. ChatViewController never computes a content offset by hand, and ChatLayout's keepContentOffsetAtBottomOnBatchUpdates pins the bottom edge across the single performBatchUpdates that both inserts the new cell and reconfigures the old one's receipt away.

One behaviour change: sending while scrolled well up the transcript now snaps to the newest message instead of the previous instant scrollToItem(0). Same destination, and the animated branch was never taken in that case.

The receipt reveal scaled

The "Delivered" line grew in with expandVertically() + scaleIn(0.95f) + fadeIn. The scale has no counterpart on iOS: ChatColumnCell un-hides a ChatReceiptLabel inside its stack view and cross-fades the text, so the line only ever changes opacity while the cell self-sizes around it. The vertical growth is the cell resizing, which is what expandVertically already models.

scaleIn is removed from the reveal. The expand, the fade, the receipt's exit, the Delivered/Read swap, and every spring spec in ChatAnimations are unchanged, since design signed off on the current tuning.