fix: broadcast full CRDT state on connect and when new user joins

CRDT broadcasts are fire-and-forget, so any updates missed during a
disconnection were permanently lost. Now when a client connects (or
reconnects), it broadcasts its full Yjs doc state. When an existing client
sees a new user join, it also broadcasts its full state. Yjs merges
handle deduplication automatically, so this converges all clients.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/editor/[projectId]/FlowchartEditor.tsx

@@ -605,6 +605,8 @@ function FlowchartEditorInner({ projectId, projectName, userId, userDisplayName,
           ...prev,
           { id: nanoid(), displayName: user.displayName, type: 'join', color: getUserColor(user.userId) },
         ])
+        // Send full CRDT state so the joining client gets caught up
+        crdtManager.broadcastFullState()
       },
       onPresenceLeave: (user) => {
         setCollaborationNotifications((prev) => [

src/lib/collaboration/crdt.ts

@@ -63,6 +63,19 @@ export class CRDTManager {
   /** Connect to a Supabase Realtime channel for outbound broadcasts */
   connectChannel(channel: RealtimeChannel): void {
     this.channel = channel
+    // Broadcast full state so other clients merge any updates they missed
+    this.broadcastFullState()
+  }
+
+  /** Broadcast the full Yjs document state to sync all connected clients */
+  broadcastFullState(): void {
+    if (!this.channel || this.isDestroyed) return
+    const state = Y.encodeStateAsUpdate(this.doc)
+    this.channel.send({
+      type: 'broadcast',
+      event: BROADCAST_EVENT,
+      payload: { update: Array.from(state) },
+    })
   }
 
   /** Apply a remote CRDT update received via broadcast */