fix(payment): enhance fee rate input validation and UI

Backend:
- Validate recharge_fee_rate: 0 ≤ rate ≤ 100, max 2 decimal places

Frontend settings:
- Add % suffix icon to fee rate input
- Enforce max=100, min=0, step=0.01 with 2 decimal precision

backend/internal/service/payment_config_service.go

@@ -272,8 +272,13 @@ func (s *PaymentConfigService) UpdatePaymentConfig(ctx context.Context, req Upda
 		}
 	}
 	if req.RechargeFeeRate != nil {
-		if math.IsNaN(*req.RechargeFeeRate) || math.IsInf(*req.RechargeFeeRate, 0) || *req.RechargeFeeRate < 0 {
-			return infraerrors.BadRequest("INVALID_RECHARGE_FEE_RATE", "recharge fee rate must be >= 0")
+		v := *req.RechargeFeeRate
+		if math.IsNaN(v) || math.IsInf(v, 0) || v < 0 || v > 100 {
+			return infraerrors.BadRequest("INVALID_RECHARGE_FEE_RATE", "recharge fee rate must be between 0 and 100")
+		}
+		// Enforce max 2 decimal places
+		if math.Round(v*100) != v*100 {
+			return infraerrors.BadRequest("INVALID_RECHARGE_FEE_RATE", "recharge fee rate allows at most 2 decimal places")
 		}
 	}
 	m := map[string]string{

frontend/src/views/admin/SettingsView.vue

@@ -2383,7 +2383,10 @@
                 </div>
                 <div>
                   <label class="input-label">{{ t('admin.settings.payment.rechargeFeeRate') }}</label>
-                  <input :value="form.payment_recharge_fee_rate || ''" @input="form.payment_recharge_fee_rate = parseFloat(($event.target as HTMLInputElement).value) || 0" type="number" step="0.01" min="0" class="input" />
+                  <div class="relative">
+                    <input :value="form.payment_recharge_fee_rate ?? ''" @input="form.payment_recharge_fee_rate = Math.min(100, Math.max(0, Math.round(parseFloat(($event.target as HTMLInputElement).value || '0') * 100) / 100))" type="number" step="0.01" min="0" max="100" class="input pr-8" />
+                    <span class="pointer-events-none absolute inset-y-0 right-0 flex items-center pr-3 text-gray-400">%</span>
+                  </div>
                   <p class="mt-0.5 text-xs text-gray-400">{{ t('admin.settings.payment.rechargeFeeRateHint') }}</p>
                   <p v-if="(Number(form.payment_recharge_fee_rate) || 0) > 0" class="mt-1 text-xs font-medium text-primary-600 dark:text-primary-400">{{ t('admin.settings.payment.rechargeFeePreview', { fee: (Number(form.payment_recharge_fee_rate) || 0).toFixed(2) }) }}</p>
                 </div>