feat: distinguish system events from user messages

System events (heartbeats, cron triggers, webhooks, channel events)
now render as subtle inline notifications instead of full user bubbles.
Detection based on content patterns ([EVENT], [cron:], [HEARTBEAT], etc.).

src/types/index.ts

@@ -6,6 +6,7 @@ export interface ChatMessage {
   blocks: MessageBlock[];
   isStreaming?: boolean;
   runId?: string;
+  isSystemEvent?: boolean;
 }
 
 export type MessageBlock =