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/stores/toast.ts

@@ -0,0 +1,34 @@
+import { create } from 'zustand'
+
+export type ToastType = 'success' | 'error' | 'info'
+
+export interface Toast {
+  id: string
+  type: ToastType
+  message: string
+  duration?: number
+}
+
+interface ToastState {
+  toasts: Toast[]
+  addToast: (t: Omit<Toast, 'id'>) => void
+  removeToast: (id: string) => void
+}
+
+export const useToastStore = create<ToastState>((set) => ({
+  toasts: [],
+
+  addToast: (t) => {
+    const id = Date.now().toString(36) + Math.random().toString(36).slice(2, 6)
+    const toast: Toast = { ...t, id }
+    set((state) => ({ toasts: [...state.toasts, toast] }))
+
+    const duration = t.duration ?? 3000
+    setTimeout(() => {
+      set((state) => ({ toasts: state.toasts.filter((x) => x.id !== id) }))
+    }, duration)
+  },
+
+  removeToast: (id) =>
+    set((state) => ({ toasts: state.toasts.filter((t) => t.id !== id) })),
+}))