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
shaw