fix(coach): move focus to the composer before hiding the jump button

Jumping to the latest message pinned the thread first, which hides the
button behind aria-hidden, and only then moved focus to the composer.
Focus now leaves the button before its state changes.

src/components/insights/coach-panel/__tests__/scroll-to-bottom-button.test.tsx

@@ -57,8 +57,10 @@ describe("<ScrollToBottomButton>", () => {
     expect(src).toContain(
       "<ScrollToBottomButton visible={!pinned} onClick={scrollToLatest} />",
     );
+    // Focus leaves the button BEFORE the state that hides it changes: a
+    // focused control under aria-hidden is announced as nothing.
     expect(src).toMatch(
-      /scrollToLatest = useCallback\([\s\S]*?wasPinnedRef\.current = true;[\s\S]*?focusCoachComposer\(\);/,
+      /scrollToLatest = useCallback\(\(\) => \{[^}]*?focusCoachComposer\(\);[^}]*?setPinned\(true\);/,
     );
   });
 });

src/components/insights/coach-panel/message-thread.tsx

@@ -394,10 +394,12 @@ export function MessageThread({
   const scrollToLatest = useCallback(() => {
     const el = scrollerRef.current;
     if (!el) return;
+    // Focus first: pinning hides this button (aria-hidden), and focus must
+    // not sit on a control that has left the accessibility tree.
+    focusCoachComposer();
     wasPinnedRef.current = true;
     setPinned(true);
     el.scrollTo({ top: el.scrollHeight, behavior: scrollBehaviorForUser() });
-    focusCoachComposer();
   }, []);
 
   // Auto-scroll on new messages OR streaming-content growth, but only