fix(ui): remove redundant sub-table header in user breakdown

The expanded user breakdown rows already align with the parent table
columns (Requests, Token, Actual, Standard), so the repeated sub-header
wastes vertical space. Remove the <thead> from UserBreakdownSubTable.

frontend/src/components/charts/UserBreakdownSubTable.vue

@@ -7,15 +7,6 @@
       {{ t('admin.dashboard.noDataAvailable') }}
     </div>
     <table v-else class="w-full text-xs">
-      <thead>
-        <tr class="text-gray-400 dark:text-gray-500">
-          <th class="py-1 pl-6 text-left">{{ t('admin.dashboard.spendingRankingUser') }}</th>
-          <th class="py-1 text-right">{{ t('admin.dashboard.requests') }}</th>
-          <th class="py-1 text-right">{{ t('admin.dashboard.tokens') }}</th>
-          <th class="py-1 text-right">{{ t('admin.dashboard.actual') }}</th>
-          <th class="py-1 pr-1 text-right">{{ t('admin.dashboard.standard') }}</th>
-        </tr>
-      </thead>
       <tbody>
         <tr
           v-for="user in items"