fix(settings): 日志保留时间移出顶部并紧凑化

通用配置卡内顺序改为 主题 → 语言 → 日志保留时间;保留时间从嵌套整卡
压缩为单行分区(标签 + 两个数字输入 + 内联保存),与主题/语言分区同族,
删去 desc 与两条 hint 冗余文案(中英同步)。

frontend/src/i18n/locales/en/settings.ts

@@ -3,11 +3,8 @@
 export default {
   retention: {
     title: 'Log retention',
-    desc: 'How long the platform keeps its own records. Loki log retention itself is configured per deployment (Component Deploy → Retention).',
-    alertLabel: 'Alert delivery history (days)',
-    alertHint: 'Expired records are purged automatically; default 7 days',
+    alertLabel: 'Alert history (days)',
     auditLabel: 'Audit logs (days)',
-    auditHint: '90–180 days recommended for compliance',
     save: 'Save',
     saving: 'Saving…',
     saved: 'Retention saved; expired records purged',