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

04 - Flows/Seller Offer Flow.md

@@ -99,37 +99,37 @@ sequenceDiagram
     autonumber
     actor S as Seller
     actor B as Buyer
-    participant FE_S as Frontend (seller)
-    participant FE_B as Frontend (buyer)
+    participant FE_S as Seller frontend
+    participant FE_B as Buyer frontend
     participant BE as Backend
     participant DB as MongoDB
     participant N as NotificationService
-    participant IO as Socket.IO
+    participant IO as SocketIO
 
-    S->>FE_S: Browse /dashboard/seller/marketplace
-    FE_S->>BE: GET /api/marketplace/purchase-requests?sellerId
-    BE-->>FE_S: filtered list
-    S->>FE_S: Open request, click "Send proposal"
-    S->>FE_S: Fill price, ETA, notes; submit
+    S->>FE_S: Browse marketplace
+    FE_S->>BE: GET /api/marketplace/purchase-requests
+    BE-->>FE_S: filtered request list
+    S->>FE_S: Open request and send offer
     FE_S->>BE: POST /api/marketplace/offers
-    BE->>DB: ensure no existing offer; check status
-    BE->>DB: SellerOffer.create({status:"pending"})
-    opt first offer on the request
-        BE->>DB: PurchaseRequest.status = "received_offers"
+    BE->>DB: Validate offer not duplicate
+    BE->>DB: Validate request status
+    BE->>DB: Create offer with status pending
+    opt request has no offers yet
+        BE->>DB: Set request status to received_offers
     end
-    BE->>N: notifyNewOfferReceived(buyer, requestId, sellerName)
-    N->>IO: emit user-{buyer} new-notification
-    BE->>IO: emit seller-{sellerId} 'new-offer'
+    BE->>N: notifyNewOfferReceived
+    N->>IO: emit notification to buyer
+    BE->>IO: emit seller new-offer
     BE-->>FE_S: 200 { offer }
-    IO-->>FE_B: new-notification (buyer's bell icon)
+    IO-->>FE_B: notify buyer bell icon
     B->>FE_B: Open request detail
     FE_B->>BE: GET /api/marketplace/offers/request/{id}
-    BE-->>FE_B: offers[]
-    alt Buyer accepts via payment
-        B->>FE_B: Click "Pay" → starts [[Payment Flow - SHKeeper]]
-        Note over BE,DB: SHKeeper webhook PAID arrives later;<br/>winning offer → accepted, others → rejected
-    else Buyer negotiates
-        B->>FE_B: Open chat → [[Negotiation Flow]]
+    BE-->>FE_B: offers
+    alt
+        B->>FE_B: Click pay to finish selected offer
+        B->>FE_B: SHKeeper webhook handles payment result
+    else
+        B->>FE_B: Open chat to negotiate
     end
 ```