feat(proxies): add account count column to proxy list

Display the number of accounts bound to each proxy in the admin proxy
management page, similar to the groups list view.

frontend/src/i18n/locales/en.ts

@@ -1561,6 +1561,7 @@ export default {
         protocol: 'Protocol',
         address: 'Address',
         status: 'Status',
+        accounts: 'Accounts',
         actions: 'Actions'
       },
       testConnection: 'Test Connection',

frontend/src/i18n/locales/zh.ts

@@ -1647,6 +1647,7 @@ export default {
         protocol: '协议',
         address: '地址',
         status: '状态',
+        accounts: '账号数',
         actions: '操作',
         nameLabel: '名称',
         namePlaceholder: '请输入代理名称',