fix(canvas/chat): instant-scroll to bottom on first mount

Reported: "right now when chat box opens it opens in the middle, but
it should be at the end of conversation."

Root cause: ChatTab.tsx:548 fires `bottomRef.scrollIntoView({ behavior:
"smooth" })` on every messages-update. On initial mount with N
messages already loaded, the smooth-scroll triggers a ~300ms animation
that any concurrent React re-render (agent push landing, theme
toggle, sidepanel resize) interrupts mid-flight, leaving the user
stuck somewhere in the middle of the conversation.

Fix: track first-mount via hasInitialScrollRef. Use behavior:"instant"
for the initial jump (deterministic, no animation interruption), then
smooth for subsequent appends (the new-message-landing visual stays).

Refs flipped on first messages.length > 0 transition, so:
- Initial open of chat tab: instant jump to bottom ✓
- New agent message arrives: smooth scroll into view ✓
- Workspace switch (ChatTab remounts): fresh hasInitialScrollRef, gets
  instant again ✓
- loadOlder prepend: anchor-restore path unchanged, still pins user's
  reading position ✓

Test plan:
- pnpm test --run ChatTab.lazyHistory.test.tsx → 8 pass (existing
  lazy-history tests untouched)
- npx tsc --noEmit clean
- Manual on hongming.moleculesai.app: open a busy chat (mac laptop,
  ~50 messages), confirm view lands at the latest bubble, not mid-
  scroll. Switch to another workspace + back → instant again.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

canvas/src/components/tabs/ChatTab.tsx

@@ -286,6 +286,14 @@ function MyChatPanel({ workspaceId, data }: Props) {
   const [error, setError] = useState<string | null>(null);
   const [confirmRestart, setConfirmRestart] = useState(false);
   const bottomRef = useRef<HTMLDivElement>(null);
+  // First-mount scroll-to-bottom needs `behavior: "instant"` — long
+  // conversations smooth-animate for ~300ms which any concurrent
+  // re-render can interrupt, leaving the user stuck mid-conversation
+  // when the chat tab opens. Subsequent appends (new agent messages)
+  // keep `smooth` for the visual "landing" feel. Flipped the first
+  // time messages.length goes positive, so a workspace switch (which
+  // remounts ChatTab) gets a fresh instant jump too.
+  const hasInitialScrollRef = useRef(false);
   // Lazy-load older history on scroll-up.
   // - containerRef = the scrollable messages viewport
   // - topRef       = sentinel above the messages list; IO observes it
@@ -545,6 +553,15 @@ function MyChatPanel({ workspaceId, data }: Props) {
       scrollAnchorRef.current = null;
       return;
     }
+    // Instant on first arrival of messages — smooth-scroll on a long
+    // conversation gets interrupted by concurrent renders and leaves
+    // the user stuck in the middle. After the first jump, subsequent
+    // appends animate as before.
+    if (!hasInitialScrollRef.current && messages.length > 0) {
+      hasInitialScrollRef.current = true;
+      bottomRef.current?.scrollIntoView({ behavior: "instant" as ScrollBehavior });
+      return;
+    }
     bottomRef.current?.scrollIntoView({ behavior: "smooth" });
   }, [messages]);