feat: add bootstrap availability check to login page with conditional UI

Add useEffect hook to fetch bootstrap status on component mount. Add bootstrapAvailable and bootstrapStatusLoaded state variables to track bootstrap endpoint availability. Hide mode toggle button when bootstrap is unavailable or status hasn't loaded yet. Add auth-brand and auth-brand-copy CSS classes to improve login page layout and branding. Add BootstrapStatus handler and BootstrapAvailable service method to expose bootstrap availability

backend/internal/httpserver/router.go

@@ -32,6 +32,7 @@ func NewRouter(jwtSecret string, handlers Handlers) http.Handler {
 	})
 
 	r.Route("/api/v1", func(r chi.Router) {
+		r.Get("/auth/bootstrap/status", handlers.Auth.BootstrapStatus)
 		r.Post("/auth/bootstrap", handlers.Auth.Bootstrap)
 		r.Post("/auth/login", handlers.Auth.Login)
 		r.Post("/auth/refresh", handlers.Auth.Refresh)