fix: round-2 audit fixes — security, code quality, and UI improvements

Security (HIGH):
- Normalize all Redis cache keys to lowercase (verifyCode, passwordReset)
- Fix verify code TTL renewal on failed attempts: use remaining TTL via
  ExpiresAt field instead of resetting to full 15-minute window
- Add 3 missing fields to diffSettings audit log (promo_code, invitation_code,
  custom_endpoints)

Code quality (MEDIUM):
- Extract filterVerifiedEmails shared helper (balance_notify_service.go)
- Add Pricing array non-empty validation for channel pricing rules
- Add platform token semantics comment in gateway_service.go
- Complete validatePlanPatch test coverage (+10 test cases)
- Replace string types with QuotaThresholdType/QuotaResetMode across frontend
- Remove duplicate getPlatformTextColor/getRateBadgeClass in ChannelsView
- Return EMAIL_NOT_FOUND error on RemoveNotifyEmail miss

UI improvements:
- Reorder cost tooltip: user billing above separator, account billing below
- Add NaN guard to accountBilled function
- Move timezone selector inline into reset-mode row (no longer standalone)

frontend/src/constants/account.ts

@@ -8,3 +8,8 @@ export type WebSearchMode = typeof WEB_SEARCH_MODE_DEFAULT | typeof WEB_SEARCH_M
 export const QUOTA_THRESHOLD_TYPE_FIXED = 'fixed' as const
 export const QUOTA_THRESHOLD_TYPE_PERCENTAGE = 'percentage' as const
 export type QuotaThresholdType = typeof QUOTA_THRESHOLD_TYPE_FIXED | typeof QUOTA_THRESHOLD_TYPE_PERCENTAGE
+
+/** Quota reset mode values */
+export const QUOTA_RESET_MODE_ROLLING = 'rolling' as const
+export const QUOTA_RESET_MODE_FIXED = 'fixed' as const
+export type QuotaResetMode = typeof QUOTA_RESET_MODE_ROLLING | typeof QUOTA_RESET_MODE_FIXED