fix(chat): keep latest-turn actions under the reply (BUG-1042)

The BUG-930 pin spacer sat on the last assistant row, pushing its sibling
.message-actions (and follow-ups / delivery card) below half a screen of
empty space. Move the same min-height onto the last turn's entry
(`:last-child:has(.message-assistant)`) with align-content: start so the
rectification grid wrap does not stretch its rows. Hook logic unchanged.

Records BUG-1043 / BUG-1044 (pre-existing, investigating) found during
browser verification.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017eEAG8HD3mm8gsKXgk8uU8

frontend/src/hooks/use-conversation-scroll-anchor.ts

@@ -84,8 +84,11 @@ export function lastTurnTail(container: HTMLElement): HTMLElement | null {
 
 export function turnHeadHeightForSpacer(head: HTMLElement | null, tail: HTMLElement | null): number {
   if (!head) return 0;
-  // The last assistant carries the min-height spacer. Measuring it as the turn
-  // head feeds the spacer back into itself (BUG-931).
+  // The last turn's entry, which starts at this assistant row, carries the
+  // min-height spacer (BUG-1042). Subtracting the row's own height would leave
+  // that entry one row short of the viewport; before BUG-1042 the row itself
+  // carried the spacer and measuring it fed the spacer back into itself
+  // (BUG-931). Either way the head-is-tail case subtracts nothing.
   if (tail && head === tail) return 0;
   return head.offsetHeight;
 }