H5: diffSettings now tracks 5 balance/quota notify fields in audit log
M15: log.Printf audit log migrated to slog.Info, removed "log" import
M14: New frontend/src/constants/account.ts with shared constants
QuotaNotifyToggle.vue uses QUOTA_THRESHOLD_TYPE_FIXED/PERCENTAGE
L2: UsageTable.vue uses BILLING_MODE_TOKEN/IMAGE from billingMode.ts