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)

backend/internal/service/email_service.go

@@ -55,6 +55,7 @@ type VerificationCodeData struct {
 	Code      string
 	Attempts  int
 	CreatedAt time.Time
+	ExpiresAt time.Time // absolute expiry; used to preserve remaining TTL when updating attempts
 }
 
 // PasswordResetTokenData represents password reset token data
@@ -263,6 +264,7 @@ func (s *EmailService) SendVerifyCode(ctx context.Context, email, siteName strin
 		Code:      code,
 		Attempts:  0,
 		CreatedAt: time.Now(),
+		ExpiresAt: time.Now().Add(verifyCodeTTL),
 	}
 	if err := s.cache.SetVerificationCode(ctx, email, data, verifyCodeTTL); err != nil {
 		return fmt.Errorf("save verify code: %w", err)
@@ -295,7 +297,11 @@ func (s *EmailService) VerifyCode(ctx context.Context, email, code string) error
 	// 验证码不匹配 (constant-time comparison to prevent timing attacks)
 	if subtle.ConstantTimeCompare([]byte(data.Code), []byte(code)) != 1 {
 		data.Attempts++
-		if err := s.cache.SetVerificationCode(ctx, email, data, verifyCodeTTL); err != nil {
+		remaining := time.Until(data.ExpiresAt)
+		if remaining <= 0 {
+			return ErrInvalidVerifyCode
+		}
+		if err := s.cache.SetVerificationCode(ctx, email, data, remaining); err != nil {
 			slog.Error("failed to update verification attempt count", "email", email, "error", err)
 		}
 		if data.Attempts >= maxVerifyCodeAttempts {