Guide Claude on securing Vaadin 25 applications with Spring Security. This skill should be used when the user asks to "add security", "add login", "create a login view", "create a login form", "use Spring Security", "secure a view", "add authentication", "add authorization", "use @RolesAllowed", "use @PermitAll", "use @AnonymousAllowed", "use @DenyAll", "use VaadinSecurityConfigurer", "add OAuth2", "use OAuth2 login", "use Google login", "use Keycloak", "use GitHub login", "add logout", "add a logout button", "use AuthenticationContext", "protect a view", "role-based access", "configure SecurityFilterChain", or needs help with view access control, login forms, OAuth2 providers, or logout handling in Vaadin Flow.