fix(group): harden peak-rate config handling and label peak windows with server timezone

Follow-up fixes to #3569 based on code audit:

- Expose server_timezone / server_utc_offset in public settings (and the
  __APP_CONFIG__ injection payload) and label every peak-window display
  with the server UTC offset, so users don't misread the billing window
  as browser-local time
- Unify CreateGroup/UpdateGroup peak-config sanitization via a single
  NormalizePeakRateConfig chokepoint: non-subscription groups always get
  peak fields cleared; unparseable window strings and negative
  multipliers are scrubbed when peak is disabled
- Replace hot-path time.Parse in PeakMultiplierAt with a manual HH:MM
  parser (accept set verified byte-for-byte identical to
  time.Parse("15:04") by exhaustive fuzzing) and reuse it in validation
- Revert the zero-behavior CalculateCost indirection churn in
  billing_service/gateway_service introduced by #3569
- Remove dead GetGroupPlatformMap and the duplicate deref helper in the
  admin handler package
- Share frontend peak formatting via utils/peak-rate.ts, unify the ×N
  label format, and move hardcoded Chinese tooltips to i18n keys

frontend/src/types/index.ts

@@ -227,6 +227,10 @@ export interface PublicSettings {
   google_oauth_enabled: boolean
   backend_mode_enabled: boolean
   version: string
+  // 服务器全局时区（IANA 名称与当前 UTC 偏移），高峰时段等服务端本地时间窗口的展示标注用；
+  // 可选：注入的 __APP_CONFIG__ 旧缓存可能缺失
+  server_timezone?: string
+  server_utc_offset?: string
   balance_low_notify_enabled: boolean
   account_quota_notify_enabled: boolean
   balance_low_notify_threshold: number