后端功能增强：全局异常处理、API控制器、JSP视图和单元测试

- 添加 GlobalExceptionHandler 全局异常处理
- 添加 ApiController REST API 控制器
- 更新 WebConfig 跨域配置和 ProductRepository 查询方法
- 新增 monitor/product-detail/profile JSP 视图页面
- 添加 FlashSaleServiceTest 秒杀服务单元测试
- 更新 application.yml 配置

flash-sale-frontend/src/components/business/CountDown.vue

@@ -0,0 +1,71 @@
+<template>
+  <div class="countdown-timer">
+    <template v-if="timeLeft > 0">
+      <el-icon class="text-red-500 mr-1"><Clock /></el-icon>
+      <span class="time-block">{{ hours.toString().padStart(2, '0') }}</span>
+      <span class="separator">:</span>
+      <span class="time-block">{{ minutes.toString().padStart(2, '0') }}</span>
+      <span class="separator">:</span>
+      <span class="time-block">{{ seconds.toString().padStart(2, '0') }}</span>
+    </template>
+    <span v-else class="text-gray-400">已结束</span>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { ref, computed, onMounted, onUnmounted } from 'vue'
+
+const props = defineProps<{
+  endTime: number
+}>()
+
+const emit = defineEmits<{
+  finish: []
+}>()
+
+const timeLeft = ref(0)
+let timer: number | null = null
+
+const hours = computed(() => Math.floor(timeLeft.value / 3600))
+const minutes = computed(() => Math.floor((timeLeft.value % 3600) / 60))
+const seconds = computed(() => timeLeft.value % 60)
+
+const updateTime = () => {
+  const now = Date.now()
+  const remaining = Math.max(0, Math.floor((props.endTime - now) / 1000))
+  timeLeft.value = remaining
+  
+  if (remaining === 0 && timer) {
+    clearInterval(timer)
+    timer = null
+    emit('finish')
+  }
+}
+
+onMounted(() => {
+  updateTime()
+  if (timeLeft.value > 0) {
+    timer = setInterval(updateTime, 1000)
+  }
+})
+
+onUnmounted(() => {
+  if (timer) {
+    clearInterval(timer)
+  }
+})
+</script>
+
+<style scoped lang="scss">
+.countdown-timer {
+  @apply flex items-center justify-center text-lg font-mono;
+  
+  .time-block {
+    @apply px-2 py-1 bg-red-50 text-red-600 rounded;
+  }
+  
+  .separator {
+    @apply mx-1 text-red-500 font-bold;
+  }
+}
+</style>