fix(perps): use markPrice fallback when limit price not yet entered

When order type is limit but no limit price is set, margin calculation
now falls back to markPrice (oracle) instead of mid price via
effectivePrice. Addresses bugbot review feedback.

Author: abretonc7s

app/components/UI/Perps/Views/PerpsOrderView/PerpsOrderView.tsx

@@ -567,10 +567,15 @@ const PerpsOrderViewContentBase: React.FC<PerpsOrderViewContentProps> = ({
 
   const marginRequired = useMemo(() => {
     if (!isLoadingMarketData && orderForm.amount) {
-      // For limit orders, use the limit price (via effectivePrice) for margin calculation.
-      // For market orders, use markPrice (oracle price) which is the standard margin basis.
-      const priceForMargin =
-        orderForm.type === 'limit' ? effectivePrice : assetData.markPrice;
+      // For limit orders with a valid limit price, use that price for margin calculation.
+      // Otherwise use markPrice (oracle price) which is the standard margin basis.
+      const hasValidLimitPrice =
+        orderForm.type === 'limit' &&
+        orderForm.limitPrice &&
+        parseFloat(orderForm.limitPrice) > 0;
+      const priceForMargin = hasValidLimitPrice
+        ? effectivePrice
+        : assetData.markPrice;
       return calculateMarginRequired({
         amount: BigNumber(priceForMargin).times(positionSize).toString(),
         leverage: orderForm.leverage,
@@ -579,6 +584,7 @@ const PerpsOrderViewContentBase: React.FC<PerpsOrderViewContentProps> = ({
   }, [
     orderForm.amount,
     orderForm.type,
+    orderForm.limitPrice,
     effectivePrice,
     assetData.markPrice,
     orderForm.leverage,