* fix(#583): align last message bottom to viewport on scroll-to-bottom
animateScrollToItem(lastIndex) only top-aligns the last item, so when the
last message is taller than the viewport its bottom is clipped below the fold.
Replace the 4 scroll-to-bottom call sites (send auto-scroll, session-switch,
auto-scroll-on-new-message, and the FAB) with a scrollToBottom() helper that
top-aligns then scrolls the exact remaining delta so the last item's bottom
edge is visible.
Using the remaining delta (not scrollOffset=Int.MAX_VALUE) avoids integer
overflow in the internal scroll-position clamp, which would wrap the offset to
0 and scroll back to the top for tall items.
* fix(#583): keep smooth animation on scroll-to-bottom when animated
agy review (PR #590) flagged that the animated path did an instant
scrollToItem then only animated the small remaining delta, causing a hard
jump on FAB / send clicks instead of a smooth scroll. Use animateScrollToItem
when animated, scrollToItem when instant.
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Co-authored-by: M57 <m57@hermes-control.app>
