fix(transaction): use eth_gasPrice RPC for Legacy transaction gas estimation

- Changed getGasPrice() to use eth_gasPrice RPC method instead of calculating
  from baseFeePerGas (which is for EIP-1559 transactions)
- Added 10% buffer to gas price to ensure transaction gets included
- Updated Home.tsx to use gasPrice instead of maxFeePerGas for display

KAVA doesn't support EIP-1559, so we must use Legacy (Type 0) transactions
with gasPrice from eth_gasPrice RPC.

🤖 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

@@ -317,8 +317,8 @@ export default function Home() {
   };
 
   // 格式化 gas 费用显示
-  const formatGasFee = (gasLimit: bigint, maxFeePerGas: bigint): string => {
-    const maxFee = gasLimit * maxFeePerGas;
+  const formatGasFee = (gasLimit: bigint, gasPrice: bigint): string => {
+    const maxFee = gasLimit * gasPrice;
     const feeKava = Number(maxFee) / 1e18;
     return feeKava.toFixed(8).replace(/\.?0+$/, '');
   };
@@ -583,7 +583,7 @@ export default function Home() {
                     <div className={styles.confirmRow}>
                       <span className={styles.confirmLabel}>预估 Gas 费</span>
                       <span className={styles.confirmValue}>
-                        ~{formatGasFee(preparedTx.gasLimit, preparedTx.maxFeePerGas)} KAVA
+                        ~{formatGasFee(preparedTx.gasLimit, preparedTx.gasPrice)} KAVA
                       </span>
                     </div>
                     <div className={styles.confirmRow}>

backend/mpc-system/services/service-party-app/src/utils/transaction.ts

@@ -335,33 +335,37 @@ export async function getNonce(address: string): Promise<number> {
 
 /**
  * 通过 RPC 获取当前 gas 价格
+ * 使用 eth_gasPrice 方法获取 Legacy 交易的 gasPrice
  */
 export async function getGasPrice(): Promise<{ maxFeePerGas: bigint; maxPriorityFeePerGas: bigint }> {
   const rpcUrl = getCurrentRpcUrl();
 
-  // 获取基础费用
-  const blockResponse = await fetch(rpcUrl, {
+  // 使用 eth_gasPrice 获取建议的 gas 价格
+  const response = await fetch(rpcUrl, {
     method: 'POST',
     headers: { 'Content-Type': 'application/json' },
     body: JSON.stringify({
       jsonrpc: '2.0',
-      method: 'eth_getBlockByNumber',
-      params: ['latest', false],
+      method: 'eth_gasPrice',
+      params: [],
       id: 1,
     }),
   });
 
-  const blockData = await blockResponse.json();
-  const baseFeePerGas = blockData.result?.baseFeePerGas
-    ? BigInt(blockData.result.baseFeePerGas)
-    : BigInt(1000000000); // 默认 1 gwei
+  const data = await response.json();
+  let gasPrice: bigint;
 
-  // 优先费用设为 1 gwei
-  const maxPriorityFeePerGas = BigInt(1000000000);
-  // 最大费用 = 基础费用 * 2 + 优先费用
-  const maxFeePerGas = baseFeePerGas * 2n + maxPriorityFeePerGas;
+  if (data.result) {
+    gasPrice = BigInt(data.result);
+    // 增加 10% 以确保交易能被打包
+    gasPrice = gasPrice * 110n / 100n;
+  } else {
+    // 默认 1 gwei
+    gasPrice = BigInt(1000000000);
+  }
 
-  return { maxFeePerGas, maxPriorityFeePerGas };
+  // 为了兼容性，同时返回 maxFeePerGas (实际上用作 gasPrice)
+  return { maxFeePerGas: gasPrice, maxPriorityFeePerGas: gasPrice };
 }
 
 /**