🤖 fix: prevent thinking block collapse on page load (#1011)

When navigating to a chat with existing thinking blocks, they would
collapse immediately after the page loaded, causing a jarring layout
shift.

**Root cause:** The `ReasoningMessage` component initialized
`isExpanded` to `true`, then a `useEffect` collapsed it when
`isStreaming` was false. For historical messages (already `isStreaming:
false`), this effect fired on mount—expanding then immediately
collapsing.

**Fix:** Initialize `isExpanded` to `message.isStreaming`. Historical
messages start collapsed; actively streaming messages start expanded and
collapse when streaming ends.

---
_Generated with `mux`_

Author: ethanndickson

src/browser/components/Messages/ReasoningMessage.tsx

@@ -14,7 +14,7 @@ interface ReasoningMessageProps {
 const REASONING_FONT_CLASSES = "font-primary text-[12px] leading-[18px]";
 
 export const ReasoningMessage: React.FC<ReasoningMessageProps> = ({ message, className }) => {
-  const [isExpanded, setIsExpanded] = useState(true);
+  const [isExpanded, setIsExpanded] = useState(message.isStreaming);
   // Track the height when expanded to reserve space during collapse transitions
   const [expandedHeight, setExpandedHeight] = useState<number | null>(null);
   const contentRef = useRef<HTMLDivElement>(null);