fix: repair Mermaid diagram syntax errors and add PRD task plan

04 - Flows/Dispute Flow.md

@@ -105,7 +105,8 @@ sequenceDiagram
     BE->>DB: Chat.create({type:"group", participants:[buyer, seller], system message})
     BE->>DB: dispute.chatId = chat._id
     BE-->>FE: { dispute }
-    FE-->>B,S: chat opens (real-time via existing chat join)
+    FE-->>B: chat opens (real-time via existing chat join)
+    FE-->>S: chat opens (real-time via existing chat join)
 
     A->>FE: Admin dashboard, click "Pick up"
     FE->>BE: POST /api/disputes/{id}/assign
@@ -130,7 +131,8 @@ sequenceDiagram
         A->>BE: split — refund X to buyer, release Y to seller
     end
     BE-->>FE: { dispute }
-    IO-->>B,S: 'new-notification' dispute resolved (planned)
+    IO-->>B: 'new-notification' dispute resolved (planned)
+    IO-->>S: 'new-notification' dispute resolved (planned)
 ```
 
 ## API calls