fix(#1297): prevent scroll jump on loadMore prepend

Two cooperating fixes:

1. overflow-anchor: none on .live — disables browser scroll anchoring
   so the manual scrollTop compensation in loadMore() is the sole
   mechanism. Without this the browser auto-adjusts scrollTop when
   rows are prepended above the viewport, then our += delta doubles
   the compensation, causing the erratic jump.

2. updateLoadMoreBtn() moved before the beforeH baseline capture —
   if histHasMore becomes false the load-more button is removed here
   rather than after the delta is applied, so the button height is
   already baked into beforeH and doesn't shift the viewport post-
   compensation.

frontend/packages/shared/src/terminal.css

@@ -36,6 +36,11 @@
   overflow-y: auto;
   max-height: 32em;
   font-family: inherit;
+  /* Disable browser scroll anchoring — the terminal manages scroll
+     position manually (snapToBottom + loadMore compensation). Scroll
+     anchoring would double-compensate scrollTop during loadMore()
+     prepends, causing an erratic jump. */
+  overflow-anchor: none;
 }
 .live.terminal {
   background: transparent;