feat: add unread message indicators on sidebar sessions

Show a cyan dot on sessions that received new messages while
viewing a different session. The dot clears when switching to
that session.

src/types/index.ts

@@ -20,6 +20,7 @@ export interface Session {
   label?: string;
   messageCount?: number;
   isActive?: boolean;
+  hasUnread?: boolean;
   totalTokens?: number;
   contextTokens?: number;
   inputTokens?: number;