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

04 - Flows/Registration Flow.md

@@ -120,7 +120,8 @@ sequenceDiagram
         BE->>IO: emit user-{refId} 'referral-signup'
     end
     BE->>DB: TempVerification.findByIdAndDelete(...)
-    BE->>BE: generate tokens; push refresh
+    BE->>BE: generate tokens
+    BE->>BE: push refresh
     BE-->>FE: 200 { user, tokens }
     FE->>FE: localStorage.setItem(accessToken, refreshToken)
     FE-->>U: Redirect /dashboard/{role}