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

04 - Flows/Google OAuth Flow.md

@@ -87,9 +87,11 @@ sequenceDiagram
     else Sign-in: user missing
         BE-->>FE: 404 USER_NOT_FOUND
     else Sign-in: ok
-        BE->>DB: user.lastLoginAt = now; back-fill avatar if blank
+        BE->>DB: set user.lastLoginAt = now
+        BE->>DB: back-fill avatar if blank
     end
-    BE->>BE: generate access + refresh; push refresh
+    BE->>BE: generate access and refresh tokens
+    BE->>BE: push refresh token
     BE-->>FE: 200 { user, tokens }
     FE->>FE: localStorage.setItem(accessToken/refreshToken)
     FE-->>U: Redirect /dashboard/{role}