feat(agent): implement streamdown for real-time message rendering in chat

web/src/components/canvas/canvas-agent-chat-ui.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useRef, useState, type ReactNode } from "react";
 import { Button, Tooltip } from "antd";
 import { ArrowUp, CheckCircle2, CircleAlert, ImagePlus, LoaderCircle, Square, UserRound, Wrench, X, XCircle } from "lucide-react";
+import { Streamdown } from "streamdown";
 
 import { isPlainEnterKey } from "@/lib/keyboard-event";
 import { canvasThemes } from "@/lib/canvas-theme";
@@ -15,6 +16,8 @@ export type CanvasAgentChatMessage = {
     meta?: string;
     detail?: unknown;
     attachments?: CanvasAgentChatAttachment[];
+    /** Present while the message is actively streaming; cleared on completion. */
+    streamId?: string;
 };
 
 const WORKING_TEXT = "working...";
@@ -46,7 +49,11 @@ export function AgentChatMessage({ item, theme, user, onRejectTool, onApproveToo
         <div className={`flex items-start gap-3 ${isUser ? "justify-end" : "justify-start"}`}>
             {!isUser ? <AgentAvatar theme={theme} /> : null}
             <div className={`min-w-0 max-w-[82%] text-sm leading-6 ${isUser ? "text-right" : "text-left"}`} style={{ color }}>
-                <div className="whitespace-pre-wrap break-words text-left">{item.text}</div>
+                {isUser ? (
+                    <div className="whitespace-pre-wrap break-words text-left">{item.text}</div>
+                ) : (
+                    <Streamdown animated isAnimating={!!item.streamId}>{item.text}</Streamdown>
+                )}
                 {item.attachments?.length ? <AgentMessageAttachments attachments={item.attachments} /> : null}
                 {item.meta ? <div className="mt-1 text-[11px] opacity-45">{item.meta}</div> : null}
             </div>