feat(计费): 支持账号计费倍率快照与统计展示

- 新增 accounts.rate_multiplier（默认 1.0，允许 0）
- 使用 usage_logs.account_rate_multiplier 记录倍率快照，避免历史回算
- 统计/导出/管理端展示账号口径费用（total_cost * account_rate_multiplier）

frontend/src/components/account/UsageProgressBar.vue

@@ -15,7 +15,13 @@
         <span class="rounded bg-gray-100 px-1.5 py-0.5 dark:bg-gray-800">
           {{ formatTokens }}
         </span>
-        <span class="rounded bg-gray-100 px-1.5 py-0.5 dark:bg-gray-800"> ${{ formatCost }} </span>
+        <span class="rounded bg-gray-100 px-1.5 py-0.5 dark:bg-gray-800"> A ${{ formatAccountCost }} </span>
+        <span
+          v-if="windowStats?.user_cost != null"
+          class="rounded bg-gray-100 px-1.5 py-0.5 dark:bg-gray-800"
+        >
+          U ${{ formatUserCost }}
+        </span>
       </div>
     </div>
 
@@ -149,8 +155,13 @@ const formatTokens = computed(() => {
   return t.toString()
 })
 
-const formatCost = computed(() => {
+const formatAccountCost = computed(() => {
   if (!props.windowStats) return '0.00'
   return props.windowStats.cost.toFixed(2)
 })
+
+const formatUserCost = computed(() => {
+  if (!props.windowStats || props.windowStats.user_cost == null) return '0.00'
+  return props.windowStats.user_cost.toFixed(2)
+})
 </script>