fix(service-party-app): serialize BigInt to string for sessionStorage

BigInt cannot be serialized by JSON.stringify. Convert gasLimit,
maxFeePerGas, maxPriorityFeePerGas, and value to strings before
storing in sessionStorage.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/mpc-system/services/service-party-app/src/pages/Home.tsx

@@ -288,13 +288,21 @@ export default function Home() {
 
       if (result.success && result.sessionId) {
         // 保存交易信息到 sessionStorage，以便签名完成后使用
-        sessionStorage.setItem(`tx_${result.sessionId}`, JSON.stringify({
-          preparedTx,
+        // 注意: BigInt 无法直接 JSON 序列化，需要转换为字符串
+        const txToStore = {
+          preparedTx: {
+            ...preparedTx,
+            gasLimit: preparedTx.gasLimit.toString(),
+            maxFeePerGas: preparedTx.maxFeePerGas.toString(),
+            maxPriorityFeePerGas: preparedTx.maxPriorityFeePerGas.toString(),
+            value: preparedTx.value.toString(),
+          },
           to: transferTo,
           amount: transferAmount,
           from: transferShare.evmAddress,
           walletName: transferShare.walletName,
-        }));
+        };
+        sessionStorage.setItem(`tx_${result.sessionId}`, JSON.stringify(txToStore));
 
         // 关闭模态框并跳转到签名会话页面
         handleCloseTransfer();