fix: prevent empty screen flash when team data temporarily unavailable

When agents.json is being rewritten by the daemon, getTeamData() returns
null and getAllData() was returning empty arrays. This payload would get
broadcast to all WebSocket clients, wiping their UI with a blank screen.

Now getAllData() returns null when team data is unavailable, and
broadcastData() skips the broadcast entirely—preserving the last valid
payload on clients. Initial WebSocket connects still send a fallback
empty structure so new clients render immediately.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: khaliqgant

packages/dashboard-server/src/server.ts

@@ -1989,7 +1989,7 @@ export async function startDashboard(
 
   const getAllData = async () => {
     const team = getTeamData();
-    if (!team) return { agents: [], messages: [], activity: [], sessions: [], summaries: [] };
+    if (!team) return null; // Signal that team data is temporarily unavailable
 
     const agentsMap = new Map<string, AgentStatus>();
     const allMessages: Message[] = await getMessages(team.agents);
@@ -2297,6 +2297,14 @@ export async function startDashboard(
   const broadcastData = async () => {
     try {
       const data = await getAllData();
+
+      // Skip broadcast when team data is temporarily unavailable (e.g., agents.json
+      // being rewritten by daemon). Preserves the last valid payload so clients
+      // don't see an empty screen flash.
+      if (!data) {
+        return;
+      }
+
       const rawPayload = JSON.stringify(data);
 
       // Guard against empty/invalid payloads
@@ -2449,7 +2457,11 @@ export async function startDashboard(
 
     try {
       const data = await getAllData();
-      const payload = JSON.stringify(data);
+
+      // If team data is temporarily unavailable, send empty-but-valid structure
+      // so the client at least renders (better than sending nothing on first connect)
+      const safeData = data ?? { agents: [], messages: [], activity: [], sessions: [], summaries: [] };
+      const payload = JSON.stringify(safeData);
 
       // Guard against empty/invalid payloads
       if (!payload || payload.length === 0) {