feat(ux): Toast 通知系统，替换内联错误/成功提示

新增 toast store 和 Toast 组件，风格保存成功/失败显示 toast，
素材生成失败显示 toast，Layout 中全局渲染。

frontend/src/components/Toast.tsx

@@ -0,0 +1,32 @@
+import { useToastStore } from '../stores/toast'
+import type { ToastType } from '../stores/toast'
+import styles from './Toast.module.css'
+
+const TYPE_CLASS: Record<ToastType, string> = {
+  success: styles.toastSuccess,
+  error: styles.toastError,
+  info: styles.toastInfo,
+}
+
+export default function ToastContainer() {
+  const { toasts, removeToast } = useToastStore()
+
+  if (toasts.length === 0) return null
+
+  return (
+    <div className={styles.container}>
+      {toasts.map((t) => (
+        <div key={t.id} className={TYPE_CLASS[t.type]}>
+          <span>{t.message}</span>
+          <button
+            className={styles.closeBtn}
+            onClick={() => removeToast(t.id)}
+            aria-label="关闭"
+          >
+            ×
+          </button>
+        </div>
+      ))}
+    </div>
+  )
+}