feat(usage): add account cost display to admin dashboard and usage pages

- Add account_cost column to dashboard aggregation tables (migration 107)
- DashboardStats: add TotalAccountCost/TodayAccountCost fields
- ModelStat/GroupStat: add AccountCost field with SQL aggregation
- GetStatsWithFilters: always return TotalAccountCost (remove accountID filter)
- Dashboard Token cards: show user(green)/cost(orange)/standard(gray)
- Usage stats card: show account cost and standard below main value
- Model/Group distribution tables: add orange cost column

frontend/src/components/admin/usage/UsageStatsCards.vue

@@ -28,17 +28,14 @@
       <div class="min-w-0 flex-1">
         <p class="text-xs font-medium text-gray-500">{{ t('usage.totalCost') }}</p>
         <p class="text-xl font-bold text-green-600">
-          ${{ ((stats?.total_account_cost ?? stats?.total_actual_cost) || 0).toFixed(4) }}
+          ${{ (stats?.total_actual_cost || 0).toFixed(4) }}
         </p>
-        <p class="text-xs text-gray-400" v-if="stats?.total_account_cost != null">
-          {{ t('usage.userBilled') }}:
-          <span class="text-gray-300">${{ (stats?.total_actual_cost || 0).toFixed(4) }}</span>
-          · {{ t('usage.standardCost') }}:
-          <span class="text-gray-300">${{ (stats?.total_cost || 0).toFixed(4) }}</span>
-        </p>
-        <p class="text-xs text-gray-400" v-else>
-          {{ t('usage.standardCost') }}:
-          <span class="line-through">${{ (stats?.total_cost || 0).toFixed(4) }}</span>
+        <p class="text-xs text-gray-400">
+          <span class="text-orange-500">${{ (stats?.total_account_cost || 0).toFixed(4) }}</span>
+          <span> {{ t('usage.accountCost') }}</span>
+          <span> · </span>
+          <span>${{ (stats?.total_cost || 0).toFixed(4) }}</span>
+          <span> {{ t('usage.standardCost') }}</span>
         </p>
       </div>
     </div>