[aidan] fix/chat-history: stop retaining unopened histories

frontend/src/shared/state/agentsSlice.ts

@@ -84,6 +84,10 @@ export interface AgentSession {
   cost_usd: number;
   tokens: { input: number; output: number };
   messages: AgentMessage[];
+  /** Compact dashboard-list metadata; full messages are fetched when a chat opens. */
+  last_message_preview?: string;
+  first_user_message?: string;
+  message_count?: number;
   pending_approvals: ApprovalRequest[];
   branches: Record<string, MessageBranch>;
   active_branch_id: string;

frontend/src/shared/state/sessionDisplay.ts

@@ -36,8 +36,13 @@ export function displayChatTitle(session: AgentSession | null | undefined): stri
     return session.name;
   }
   const firstUserMsg = session.messages?.find((m) => m.role === 'user');
-  if (firstUserMsg && typeof firstUserMsg.content === 'string') {
-    const truncated = truncateForTitle(firstUserMsg.content);
+  const firstUserContent = firstUserMsg && typeof firstUserMsg.content === 'string'
+    ? firstUserMsg.content
+    : session.messages.length === 0
+      ? session.first_user_message
+      : undefined;
+  if (firstUserContent) {
+    const truncated = truncateForTitle(firstUserContent);
     if (truncated) return truncated;
   }
   return session.mode === 'view-builder' ? 'Untitled App' : SESSION_NAME_PLACEHOLDER;