fix(stock): 브로커 총 매입 금액을 매입가 단순 합계로 수정

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/pages/stock/hooks/usePortfolio.js

@@ -76,7 +76,8 @@ export default function usePortfolio() {
         for (const item of items) {
             const qty = item.quantity ?? 0;
             const purchase = item.purchase_price ?? item.avg_price ?? 0;
-            totalBuy += purchase * qty;
+            // 총 매입 = 종목별 매입가의 단순 합 (수량 미곱산)
+            totalBuy += purchase;
             totalCostBasis += (item.avg_price ?? 0) * qty;
             if (item.eval_amount != null) totalEvalAmt += item.eval_amount;
             else hasNullPrice = true;