fix(toaster): render error toasts on a red surface

Error toasts previously used the same neutral popover background as
success toasts, so a failure (e.g. a save while the backend is down)
read like a normal 'Canvas saved'. Set sonner's per-type --error-bg/
--error-text/--error-border so errors stand out in red.

frontend/src/components/ui/sonner.tsx

@@ -34,6 +34,11 @@ const Toaster = ({ ...props }: ToasterProps) => {
           "--normal-text": "var(--popover-foreground)",
           "--normal-border": "var(--border)",
           "--border-radius": "var(--radius)",
+          // Errors use a red surface so failures (e.g. a failed save while the
+          // backend is down) stand out instead of reading like a normal toast.
+          "--error-bg": "#f85149",
+          "--error-text": "#ffffff",
+          "--error-border": "#da3633",
         } as React.CSSProperties
       }
       toastOptions={{