fix: 替换通知图标为统一的Icon组件样式

- 移除自定义SVG图标，使用Icon组件的bell图标
- 确保通知图标与系统设置、成就、签到等图标风格统一
- 修复类型检查错误

frontend/src/components/NotificationBell.vue

@@ -2,19 +2,7 @@
   <a-dropdown :trigger="['click']" placement="bottomRight">
     <div class="notification-bell" @click="handleClick">
       <a-badge :count="unreadCount" :overflow-count="99">
-        <svg
-          class="bell-icon"
-          viewBox="0 0 24 24"
-          aria-hidden="true"
-          focusable="false"
-        >
-          <path
-            d="M12 2.25a4.5 4.5 0 0 0-4.5 4.5V9c0 .414-.168.81-.467 1.098L5.21 11.82a1.125 1.125 0 0 0 .79 1.93h12a1.125 1.125 0 0 0 .79-1.93l-1.823-1.722A1.5 1.5 0 0 1 16.5 9V6.75a4.5 4.5 0 0 0-4.5-4.5Zm0 18.375a3.375 3.375 0 0 0 3.182-2.25H8.818A3.375 3.375 0 0 0 12 20.625Z"
-            fill="currentColor"
-            stroke="currentColor"
-            stroke-width="0"
-          />
-        </svg>
+        <Icon name="bell" size="md" />
       </a-badge>
     </div>
     
@@ -137,7 +125,7 @@ const handleClick = () => {
 // 点击通知项
 const handleNotificationClick = async (notification: Notification) => {
   // 标记为已读
-  if (notification.isRead === 0) {
+  if (notification.isRead === 0 && currentUserId.value) {
     try {
       await markAsRead(notification.id, currentUserId.value)
       loadUnreadCount()
@@ -160,6 +148,10 @@ const handleViewAll = () => {
 
 // 全部已读
 const handleMarkAllRead = async () => {
+  if (!currentUserId.value) {
+    return
+  }
+  
   try {
     const count = await markAllAsRead(currentUserId.value)
     message.success(`已标记 ${count} 条消息为已读`)
@@ -278,15 +270,6 @@ onUnmounted(() => {
     background: $bg-card;
     color: $text-primary;
   }
-  
-  .bell-icon {
-    width: 18px;
-    height: 18px;
-    display: inline-block;
-    fill: currentColor;
-    stroke: currentColor;
-    stroke-width: 0;
-  }
 }
 
 .notification-dropdown {