fix(usage): show account cost inline under cost column, remove separate column

- Cost cell: change gray "A $xxx" to orange "成本 $xxx" with i18n
- Remove standalone account_cost column from column settings (redundant)

frontend/src/views/admin/UsageView.vue

@@ -533,7 +533,6 @@ const allColumns = computed(() => [
   { key: 'billing_mode', label: t('admin.usage.billingMode'), sortable: false },
   { key: 'tokens', label: t('usage.tokens'), sortable: false },
   { key: 'cost', label: t('usage.cost'), sortable: false },
-  { key: 'account_cost', label: t('usage.accountCost'), sortable: false },
   { key: 'first_token', label: t('usage.firstToken'), sortable: false },
   { key: 'duration', label: t('usage.duration'), sortable: false },
   { key: 'created_at', label: t('usage.time'), sortable: true },