feat: Antigravity 端点签名迁移至 gemini:chat，流式 usage 提取重构与请求详情自动刷新

- Antigravity 端点签名从 gemini:cli 统一为 gemini:chat，保留向后兼容，含 DB 迁移
- 流式处理中 usage/completion/text 提取抽离为 _update_ctx_from_provider_event，
  支持 envelope 解包后提取，避免格式转换流程中重复计数
- 新增 has_format_conversion 属性，区分真正的格式转换与 envelope rewrite，
  修正 usage 展示层的格式转换标记
- 请求详情抽屉对未完成请求支持自动轮询刷新，关闭时自动停止
- 按次计费样式调整；实际成本仅在倍率非 1.0 时显示；缓存日志降级为 trace

frontend/src/features/usage/components/HorizontalRequestTimeline.vue

@@ -642,19 +642,28 @@ const navigateGroup = (direction: number) => {
 }
 
 // 加载请求追踪数据
-const loadTrace = async () => {
+const isSilentRefresh = ref(false)
+const loadTrace = async (silent = false) => {
   if (!props.requestId) return
 
-  loading.value = true
+  isSilentRefresh.value = silent
+
+  if (!silent) {
+    loading.value = true
+  }
   error.value = null
 
   try {
     trace.value = await requestTraceApi.getRequestTrace(props.requestId)
   } catch (err: any) {
-    error.value = err.response?.data?.detail || err.message || '加载失败'
+    if (!silent) {
+      error.value = err.response?.data?.detail || err.message || '加载失败'
+    }
     log.error('加载请求追踪失败:', err)
   } finally {
-    loading.value = false
+    if (!silent) {
+      loading.value = false
+    }
   }
 }
 
@@ -662,6 +671,12 @@ const loadTrace = async () => {
 watch(groupedTimeline, (newGroups) => {
   if (!newGroups || newGroups.length === 0) return
 
+  // 静默刷新时不重置选中状态
+  if (isSilentRefresh.value) {
+    isSilentRefresh.value = false
+    return
+  }
+
   // 查找成功的组
   const successIdx = newGroups.findIndex(g => g.primaryStatus === 'success')
   if (successIdx >= 0) {
@@ -712,6 +727,8 @@ watch(() => props.requestId, () => {
   loadTrace()
 }, { immediate: true })
 
+defineExpose({ refresh: () => loadTrace(true) })
+
 // 格式化时间（详细）
 const formatTime = (dateStr: string) => {
   const date = new Date(dateStr)