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/UsageRecordsTable.vue

@@ -193,7 +193,7 @@
           <div class="flex flex-col items-end flex-shrink-0">
             <span class="text-xs text-primary font-medium">{{ formatCurrency(record.cost || 0) }}</span>
             <span
-              v-if="showActualCost && record.actual_cost !== undefined"
+              v-if="showActualCost && record.actual_cost !== undefined && record.rate_multiplier && record.rate_multiplier !== 1.0"
               class="text-[10px] text-muted-foreground"
             >{{ formatCurrency(record.actual_cost) }}</span>
           </div>
@@ -562,7 +562,7 @@
             <div class="flex flex-col items-end text-xs gap-0.5">
               <span class="text-primary font-medium">{{ formatCurrency(record.cost || 0) }}</span>
               <span
-                v-if="showActualCost && record.actual_cost !== undefined"
+                v-if="showActualCost && record.actual_cost !== undefined && record.rate_multiplier && record.rate_multiplier !== 1.0"
                 class="text-muted-foreground"
               >
                 {{ formatCurrency(record.actual_cost) }}