Play screen UX: sticky header + fix streaming autoscroll

- Pin the play header (Plot/Memory/Scripts/Insights toggles) just below
  the top nav so they stay reachable without scrolling back up to the
  first message.
- Fix streaming autoscroll: snap to the real document bottom instantly
  instead of smooth-scrolling to storyEndRef. That ref sits above the
  sticky composer, so block:'end' stopped short — hiding the latest line
  behind the input bar and yanking the reader back up when they scrolled
  down. Smooth behavior also never settled at per-token speed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TEpWMjnfPqzZ13nPMoGhs5

frontend/src/pages/Play.jsx

@@ -444,8 +444,12 @@ export default function Play() {
   }, [])
 
   useEffect(() => {
+    // Snap to the real document bottom (below the sticky composer), not to
+    // storyEndRef — that ref sits above the composer, so block:'end' would
+    // stop short and fight a reader scrolling down. Instant, not smooth: at
+    // streaming speed a queued smooth animation never settles.
     if (pinnedRef.current) {
-      storyEndRef.current?.scrollIntoView({ behavior: 'smooth', block: 'end' })
+      window.scrollTo({ top: document.documentElement.scrollHeight })
     }
   }, [actions, streaming, reasoningStream])