fix(ui): stabilize streaming message/tool rendering

Avoid remounting message blocks on part updates so tool call UI state persists. Render tool/message content from store and stabilize tool output scrolling during streaming.

packages/ui/src/components/message-part.tsx

@@ -25,6 +25,13 @@ interface MessagePartProps {
   const isAssistantMessage = () => props.messageType === "assistant"
   const textContainerClass = () => (isAssistantMessage() ? "message-text message-text-assistant" : "message-text")
 
+  const shouldHideTextPart = () => {
+    const part = props.part
+    if (!part || part.type !== "text") return false
+    // Keep optimistic user prompts visible; hide synthetic assistant text.
+    return Boolean((part as any).synthetic) && props.messageType !== "user"
+  }
+
   const plainTextContent = () => {
     const part = props.part
 
@@ -94,7 +101,7 @@ interface MessagePartProps {
   return (
     <Switch>
       <Match when={partType() === "text"}>
-        <Show when={!(props.part.type === "text" && props.part.synthetic) && partHasRenderableText(props.part)}>
+        <Show when={!shouldHideTextPart() && partHasRenderableText(props.part)}>
           <div class={textContainerClass()}>
             <Show
                when={isAssistantMessage()}