fix: error toasts persist until manually dismissed

Error variant toasts no longer auto-close after 5 seconds. Duration 0
means "no auto-dismiss". Other variants unchanged.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/design-system/composites/Toast/Toast.tsx

@@ -81,8 +81,9 @@ export function ToastProvider({ children }: { children: ReactNode }) {
   const toast = useCallback(
     (options: ToastOptions): string => {
       const id = `toast-${Date.now()}-${Math.random().toString(36).slice(2, 7)}`
-      const duration = options.duration ?? DEFAULT_DURATION
       const variant = options.variant ?? 'info'
+      // Error toasts persist until manually dismissed; others auto-close after DEFAULT_DURATION
+      const duration = options.duration ?? (variant === 'error' ? 0 : DEFAULT_DURATION)
 
       const newToast: ToastItem = {
         id,
@@ -99,11 +100,13 @@ export function ToastProvider({ children }: { children: ReactNode }) {
         return next.slice(-MAX_TOASTS)
       })
 
-      // Schedule auto-dismiss
-      const timer = setTimeout(() => {
-        dismiss(id)
-      }, duration)
-      timersRef.current.set(id, timer)
+      // Schedule auto-dismiss (duration 0 = persist until manual dismiss)
+      if (duration > 0) {
+        const timer = setTimeout(() => {
+          dismiss(id)
+        }, duration)
+        timersRef.current.set(id, timer)
+      }
 
       return id
     },