feat: optimistic message rendering with send status indicators

- User messages appear instantly with 'sending' state (dimmed, clock icon)
- Transitions to 'sent' (checkmark) when server acknowledges
- Shows error state (alert icon, retry visible) if send fails
- Applied to both primary and secondary sessions

src/types/index.ts

@@ -8,6 +8,8 @@ export interface ChatMessage {
   runId?: string;
   isSystemEvent?: boolean;
   metadata?: Record<string, unknown>;
+  /** Optimistic send status for user messages */
+  sendStatus?: 'sending' | 'sent' | 'error';
 }
 
 export type MessageBlock =