rewrite: toast stacking with daisyui toast container

- module-level reactive toast list in composables/toast (setToast signature unchanged, 13 call sites untouched)
- multiple toasts stack simultaneously in daisyUI toast container, each auto-dismisses (3s) with close button
- TransitionGroup enter/leave animation (transform/opacity, honors reduced-motion)
- drop provide/inject queue that blocked consecutive toasts

frontend/src/composables/toast.ts

@@ -1,24 +1,35 @@
-import { inject } from 'vue'
-import type { InjectionKey } from 'vue'
+import { ref } from 'vue'
 
 export type ToastType = 'info' | 'success' | 'error'
 
-export type ToastMessage = {
-  message: string
-  type?: ToastType
-  duration?: number
+export type ToastItem = {
+    id: number
+    message: string
+    type: ToastType
+    duration: number
 }
 
-export type ToastContext = {
-  setToast: (message: string, type?: ToastType, duration?: number) => void
+// 模块级共享状态：所有活动 toast 堆叠展示，互不阻塞
+const toasts = ref<ToastItem[]>([])
+let seed = 0
+
+function dismiss(id: number) {
+    toasts.value = toasts.value.filter(t => t.id !== id)
 }
 
-export const ToastKey: InjectionKey<ToastContext> = Symbol('toast')
+export function setToast(message: string, type: ToastType = 'info', duration = 3000) {
+    const id = ++seed
+    toasts.value.push({ id, message, type, duration })
+    if (duration > 0) {
+        setTimeout(() => dismiss(id), duration)
+    }
+}
 
-export function useToast(): ToastContext {
-  const ctx = inject(ToastKey)
-  if (!ctx) {
-    throw new Error('ToastContext 未提供：请在 App 根组件 provide(ToastKey, ...)')
-  }
-  return ctx
+export function useToast() {
+    return { setToast }
+}
+
+// 供 Toast 组件渲染使用
+export function useToasts() {
+    return { toasts, dismiss }
 }