Ports the usePinToSend half of #5386 to the Vue package, which is a
documented 1:1 parity port and retained the shrink-only ResizeObserver.
When content below the anchored user message loses height (suggestions
swap, input resize), the spacer now grows back so total scrollable space
below the bubble stays constant and the message stays pinned (#5355).
Vue has no scroll-to-bottom button, so the listener half of #5386 does
not apply.