修复：使用 Bell 图标替换不存在的 Megaphone 图标

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/layouts/AppLayout.vue

@@ -4,6 +4,7 @@ import { useRouter } from "vue-router";
 import { ElMessage } from "element-plus";
 import {
   ArrowDown,
+  Bell,
   ChatDotRound,
   CirclePlus,
   Coin,
@@ -12,7 +13,6 @@ import {
   Finished,
   House,
   InfoFilled,
-  Megaphone,
   Search,
   Service,
   Shop,
@@ -34,7 +34,7 @@ const navItems = [
   { label: "我的订单", to: "/orders", icon: Tickets },
   { label: "消息", to: "/messages", icon: ChatDotRound },
   { label: "钱包", to: "/wallet", icon: Wallet },
-  { label: "公告", to: "/announcements", icon: Megaphone },
+  { label: "公告", to: "/announcements", icon: Bell },
 ];
 const buyerServiceItems = [
   { label: "待支付", icon: Coin, to: { path: "/orders", query: { tab: "pending_payment" } } },