fix(frontend): 修复DataTable无限循环和i18n邮箱解析错误

- 修复DataTable组件watch监听actionsExpanded导致的无限循环卡死问题
- 为AccountsView和UsersView添加actionsCount属性启用操作列展开功能
- 修复i18n翻译中邮箱地址的@符号未转义导致的编译错误

frontend/src/i18n/locales/en.ts

@@ -1351,12 +1351,12 @@ export default {
         port: 'SMTP Port',
         portPlaceholder: '587',
         username: 'SMTP Username',
-        usernamePlaceholder: 'your-email@gmail.com',
+        usernamePlaceholder: "your-email{'@'}gmail.com",
         password: 'SMTP Password',
         passwordPlaceholder: '********',
         passwordHint: 'Leave empty to keep existing password',
         fromEmail: 'From Email',
-        fromEmailPlaceholder: 'noreply@example.com',
+        fromEmailPlaceholder: "noreply{'@'}example.com",
         fromName: 'From Name',
         fromNamePlaceholder: 'Sub2API',
         useTls: 'Use TLS',
@@ -1366,7 +1366,7 @@ export default {
         title: 'Send Test Email',
         description: 'Send a test email to verify your SMTP configuration',
         recipientEmail: 'Recipient Email',
-        recipientEmailPlaceholder: 'test@example.com',
+        recipientEmailPlaceholder: "test{'@'}example.com",
         sendTestEmail: 'Send Test Email',
         sending: 'Sending...',
         enterRecipientHint: 'Please enter a recipient email address'

frontend/src/i18n/locales/zh.ts

@@ -1549,12 +1549,12 @@ export default {
         port: 'SMTP 端口',
         portPlaceholder: '587',
         username: 'SMTP 用户名',
-        usernamePlaceholder: 'your-email@gmail.com',
+        usernamePlaceholder: "your-email{'@'}gmail.com",
         password: 'SMTP 密码',
         passwordPlaceholder: '********',
         passwordHint: '留空以保留现有密码',
         fromEmail: '发件人邮箱',
-        fromEmailPlaceholder: 'noreply@example.com',
+        fromEmailPlaceholder: "noreply{'@'}example.com",
         fromName: '发件人名称',
         fromNamePlaceholder: 'Sub2API',
         useTls: '使用 TLS',
@@ -1564,7 +1564,7 @@ export default {
         title: '发送测试邮件',
         description: '发送测试邮件以验证 SMTP 配置',
         recipientEmail: '收件人邮箱',
-        recipientEmailPlaceholder: 'test@example.com',
+        recipientEmailPlaceholder: "test{'@'}example.com",
         sendTestEmail: '发送测试邮件',
         sending: '发送中...',
         enterRecipientHint: '请输入收件人邮箱地址'