fix(i18n): escape at-sign in email whitelist placeholder

Escape the at-sign in email whitelist placeholder to avoid Vue I18n linked message parsing errors on the settings page.

frontend/src/i18n/locales/en.ts

@@ -5301,7 +5301,7 @@ export default {
         emailSuffixWhitelist: 'Email Domain Whitelist',
         emailSuffixWhitelistHint:
           "Only email addresses from the specified domains can register (for example, {'@'}qq.com, {'@'}gmail.com, *.edu.cn)",
-        emailSuffixWhitelistPlaceholder: '@example.com, *.edu.cn',
+        emailSuffixWhitelistPlaceholder: "{'@'}example.com, *.edu.cn",
         emailSuffixWhitelistInputHint: 'Leave empty for no restriction. Use *.edu.cn to match edu.cn and its subdomains.',
         promoCode: 'Promo Code',
         promoCodeHint: 'Allow users to use promo codes during registration',

frontend/src/i18n/locales/zh.ts

@@ -5464,7 +5464,7 @@ export default {
         emailSuffixWhitelist: '邮箱域名白名单',
         emailSuffixWhitelistHint:
           "仅允许使用指定域名的邮箱注册账号（例如 {'@'}qq.com, {'@'}gmail.com, *.edu.cn）",
-        emailSuffixWhitelistPlaceholder: '@example.com, *.edu.cn',
+        emailSuffixWhitelistPlaceholder: "{'@'}example.com, *.edu.cn",
         emailSuffixWhitelistInputHint: '留空则不限制。使用 *.edu.cn 可匹配 edu.cn 及其子域名。',
         promoCode: '优惠码',
         promoCodeHint: '允许用户在注册时使用优惠码',