refactor(ops): 优化阈值检查系统和布局

阈值检查系统优化：
- 引入三级阈值系统（normal/warning/critical）
- 统一阈值判断逻辑，支持警告和严重两个级别
- 移除硬编码的 TTFT 颜色判断，改用阈值配置
- 新增 getThresholdColorClass 统一颜色映射

布局优化：
- 优化详细指标在卡片内的响应式布局
- 改进宽屏下的卡片布局显示
- 优化指标数值的对齐和间距

frontend/src/views/admin/ops/OpsDashboard.vue

@@ -693,8 +693,8 @@ onMounted(async () => {
 
 async function loadThresholds() {
   try {
-    const settings = await opsAPI.getAlertRuntimeSettings()
-    metricThresholds.value = settings.thresholds || null
+    const thresholds = await opsAPI.getMetricThresholds()
+    metricThresholds.value = thresholds || null
   } catch (err) {
     console.warn('[OpsDashboard] Failed to load thresholds', err)
     metricThresholds.value = null