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/pages/GeneratePage.tsx

@@ -2,6 +2,7 @@ import { useEffect } from 'react'
 import { useNavigate, useParams } from 'react-router-dom'
 import { useTaskStore } from '../stores/task'
 import { useGenerationStore } from '../stores/generation'
+import { useToastStore } from '../stores/toast'
 import GenerateForm from '../components/GenerateForm'
 import ProgressBar from '../components/ProgressBar'
 
@@ -9,6 +10,7 @@ export default function GeneratePage() {
   const { projectId = 'proj-default' } = useParams()
   const navigate = useNavigate()
   const { assetType, reset: resetTask } = useTaskStore()
+  const addToast = useToastStore(s => s.addToast)
   const {
     status,
     stage,
@@ -25,6 +27,13 @@ export default function GeneratePage() {
     return () => resetGeneration()
   }, [resetGeneration])
 
+  // 失败时显示 toast
+  useEffect(() => {
+    if (status === 'failed') {
+      addToast({ type: 'error', message: '素材生成失败，请重试' })
+    }
+  }, [status, addToast])
+
   // 完成后自动跳转
   useEffect(() => {
     if (status === 'completed' && taskId) {