fix(i18n): rename proxy fallback label to expiry fallback

frontend/src/i18n/locales/en/admin/resources.ts

@@ -231,7 +231,7 @@ export default {
       nDays: '{days}d',
       expiryDaysPlaceholder: 'Custom days, empty = never',
       expiryWarnDays: 'Expiry warning (days)',
-      fallbackMode: 'Failure fallback',
+      fallbackMode: 'Expiry fallback',
       fallbackNone: 'No fallback',
       fallbackProxy: 'Backup proxy',
       fallbackDirect: 'Direct connection',

frontend/src/i18n/locales/zh/admin/resources.ts

@@ -229,7 +229,7 @@ export default {
       nDays: '{days} 天',
       expiryDaysPlaceholder: '自定义天数，留空 = 永不过期',
       expiryWarnDays: '到期提醒提前天数',
-      fallbackMode: '失败回退',
+      fallbackMode: '到期回退',
       fallbackNone: '不回退',
       fallbackProxy: '指定备用代理',
       fallbackDirect: '回退直连',