feat(api-key): 增加 API Key 上次使用时间并补齐测试

frontend/src/i18n/locales/en.ts

@@ -478,6 +478,7 @@ export default {
     today: 'Today',
     total: 'Total',
     quota: 'Quota',
+    lastUsedAt: 'Last Used',
     useKey: 'Use Key',
     useKeyModal: {
       title: 'Use API Key',

frontend/src/i18n/locales/zh.ts

@@ -479,6 +479,7 @@ export default {
     today: '今日',
     total: '累计',
     quota: '额度',
+    lastUsedAt: '上次使用时间',
     useKey: '使用密钥',
     useKeyModal: {
       title: '使用 API 密钥',

frontend/src/views/user/KeysView.vue

@@ -159,6 +159,13 @@
             </span>
           </template>
 
+          <template #cell-last_used_at="{ value }">
+            <span v-if="value" class="text-sm text-gray-500 dark:text-dark-400">
+              {{ formatDateTime(value) }}
+            </span>
+            <span v-else class="text-sm text-gray-400 dark:text-dark-500">-</span>
+          </template>
+
           <template #cell-created_at="{ value }">
             <span class="text-sm text-gray-500 dark:text-dark-400">{{ formatDateTime(value) }}</span>
           </template>
@@ -738,6 +745,7 @@ const columns = computed<Column[]>(() => [
   { key: 'usage', label: t('keys.usage'), sortable: false },
   { key: 'expires_at', label: t('keys.expiresAt'), sortable: true },
   { key: 'status', label: t('common.status'), sortable: true },
+  { key: 'last_used_at', label: t('keys.lastUsedAt'), sortable: true },
   { key: 'created_at', label: t('keys.created'), sortable: true },
   { key: 'actions', label: t('common.actions'), sortable: false }
 ])