Add empty state message for role assignment

- Display helpful message when no custom roles available
- Clarify that system roles are auto-assigned
- Add noCustomRolesToAssign translation in English and Chinese

src/locales/en/translation.json

@@ -1887,6 +1887,7 @@
     "failedToRemoveRole": "Failed to remove role",
     "cannotRemoveSystemRole": "Cannot remove system role",
     "cannotShareWithSelf": "Cannot share host with yourself",
+    "noCustomRolesToAssign": "No custom roles available. System roles are auto-assigned.",
     "auditLogs": "Audit Logs",
     "viewAuditLogs": "View Audit Logs",
     "action": "Action",

src/locales/zh/translation.json

@@ -1748,6 +1748,7 @@
     "failedToRemoveRole": "移除角色失败",
     "cannotRemoveSystemRole": "无法移除系统角色",
     "cannotShareWithSelf": "不能与自己共享主机",
+    "noCustomRolesToAssign": "没有可用的自定义角色。系统角色已自动分配。",
     "auditLogs": "审计日志",
     "viewAuditLogs": "查看审计日志",
     "action": "操作",

src/ui/desktop/admin/AdminSettings.tsx

@@ -1787,6 +1787,15 @@ export function AdminSettings({
                         {t(role.displayName)}
                       </Button>
                     ))}
+                  {availableRoles.filter(
+                    (role) =>
+                      !role.isSystem &&
+                      !userRoles.some((ur) => ur.roleId === role.id),
+                  ).length === 0 && (
+                    <p className="text-sm text-muted-foreground">
+                      {t("rbac.noCustomRolesToAssign")}
+                    </p>
+                  )}
                 </div>
               </div>
             </div>