remove hardcoded box colours, remove text about historical auth repo

docs/web/diagrams/auth/confirm-user.mmd

@@ -2,21 +2,19 @@ sequenceDiagram
     actor User
     participant API as API Controller
     participant Mediator as MediatR pipeline<br/>(ValidationBehavior)
-    box rgb(241,243,234) Handler Layer
+    box Handler Layer
         participant ConfirmHandler as ConfirmUserHandler
         participant UserMgr as UserManager(ApplicationUser)
         participant TokenSvc as TokenService
     end
-    box rgb(235,236,227) Infrastructure Layer
+    box Infrastructure Layer
         participant JWT as JWT Infrastructure
     end
-    box rgb(219,238,221) Repository / Cross-slice
+    box Repository / Cross-slice
         participant UserStore as DapperUserStore<br/>(IUserStore/IUserPasswordStore/IUserEmailStore)
     end
     participant DB as SQL Server
 
-    Note over UserMgr: ASP.NET Core Identity's UserManager replaces the deleted AuthRepository — handlers delegate to UserManager, which calls DapperUserStore (persistence) and Argon2PasswordHasher.
-
     User->>API: POST /api/auth/confirm?token={token} (requires Bearer access token)
     activate API
     API->>Mediator: Send(ConfirmUserCommand(token))