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/composables/useQuotaNotifyState.ts

@@ -1,6 +1,6 @@
 import { reactive, ref } from 'vue'
 import { adminAPI } from '@/api/admin'
-import { QUOTA_THRESHOLD_TYPE_FIXED } from '@/constants/account'
+import { QUOTA_THRESHOLD_TYPE_FIXED, type QuotaThresholdType } from '@/constants/account'
 
 export const QUOTA_NOTIFY_DIMS = ['daily', 'weekly', 'total'] as const
 export type QuotaNotifyDim = (typeof QUOTA_NOTIFY_DIMS)[number]
@@ -8,7 +8,7 @@ export type QuotaNotifyDim = (typeof QUOTA_NOTIFY_DIMS)[number]
 interface DimState {
   enabled: boolean | null
   threshold: number | null
-  thresholdType: string | null
+  thresholdType: QuotaThresholdType | null
 }
 
 export function useQuotaNotifyState() {
@@ -34,7 +34,7 @@ export function useQuotaNotifyState() {
     for (const d of QUOTA_NOTIFY_DIMS) {
       state[d].enabled = (extra?.[`quota_notify_${d}_enabled`] as boolean) ?? null
       state[d].threshold = (extra?.[`quota_notify_${d}_threshold`] as number) ?? null
-      state[d].thresholdType = (extra?.[`quota_notify_${d}_threshold_type`] as string) ?? null
+      state[d].thresholdType = (extra?.[`quota_notify_${d}_threshold_type`] as QuotaThresholdType) ?? null
     }
   }