feat: update toast close button with SVG icon and improve styling

webapp/src/components/ToastHost.tsx

@@ -12,8 +12,10 @@ export default function ToastHost({ toasts, onClose }: ToastHostProps) {
       {toasts.map((toast) => (
         <li key={toast.id} className={`toast-item ${toast.type}`}>
           <div className="toast-text">{toast.text}</div>
-          <button type="button" className="toast-close" onClick={() => onClose(toast.id)}>
-            x
+          <button type="button" className="toast-close" onClick={() => onClose(toast.id)} aria-label="关闭通知">
+            <svg width="14" height="14" viewBox="0 0 14 14" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" aria-hidden="true">
+              <path d="M3 3l8 8M11 3l-8 8" />
+            </svg>
           </button>
           <div className="toast-progress" />
         </li>