ui(users): banner explaining the disabled-username re-enable flow

internal/server/http/ui_users.go

@@ -153,6 +153,11 @@ type userFormPage struct {
 	SetupURL   string
 	SetupExpAt time.Time
 	Error      string
+	// Reenable is set when the admin landed here because they tried
+	// to add a username that already exists (disabled). Triggers a
+	// banner on the edit page explaining why and steering them at
+	// the Re-enable button. See handleUIUserNewPost's collision branch.
+	Reenable bool
 }
 
 func (s *Server) handleUIUserNewGet(w stdhttp.ResponseWriter, r *stdhttp.Request) {
@@ -290,6 +295,7 @@ func (s *Server) handleUIUserEditGet(w stdhttp.ResponseWriter, r *stdhttp.Reques
 		Mode: "edit", ID: target.ID, Username: target.Username,
 		Email: em, Role: string(target.Role),
 		Disabled: target.DisabledAt != nil,
+		Reenable: r.URL.Query().Get("reenable") == "1",
 	}
 	_ = s.deps.UI.Render(w, "user_edit", view)
 }