fix(blockchain): read token decimals from contract instead of hardcoding

The balance display was incorrect because decimals were hardcoded to 18,
but USDT uses 6 decimals. Now reads decimals() from the ERC20 contract.

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

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

backend/services/blockchain-service/src/api/controllers/deposit.controller.ts

@@ -78,8 +78,8 @@ export class DepositController {
           chainType: chainTypeStr,
           address: addressStr,
           balance: balance.usdtBalance,
-          rawBalance: this.toRawBalance(balance.usdtBalance, this.getDecimals(chainTypeStr)),
-          decimals: this.getDecimals(chainTypeStr),
+          rawBalance: balance.usdtRawBalance,
+          decimals: balance.usdtDecimals,
         };
 
         if (chainTypeStr === ChainTypeEnum.KAVA) {
@@ -99,25 +99,4 @@ export class DepositController {
 
     return response;
   }
-
-  private getDecimals(chainType: string): number {
-    // USDT decimals by chain
-    switch (chainType) {
-      case ChainTypeEnum.KAVA:
-        return 18; // Our test USDT on KAVA uses 18 decimals
-      case ChainTypeEnum.BSC:
-        return 18; // Standard USDT on BSC
-      default:
-        return 18;
-    }
-  }
-
-  private toRawBalance(formattedBalance: string, decimals: number): string {
-    try {
-      const value = parseFloat(formattedBalance);
-      return BigInt(Math.floor(value * Math.pow(10, decimals))).toString();
-    } catch {
-      return '0';
-    }
-  }
 }

backend/services/blockchain-service/src/application/services/balance-query.service.ts

@@ -8,6 +8,8 @@ export interface BalanceResult {
   chainType: string;
   address: string;
   usdtBalance: string;
+  usdtRawBalance: string;
+  usdtDecimals: number;
   nativeBalance: string;
   nativeSymbol: string;
 }
@@ -39,6 +41,8 @@ export class BalanceQueryService {
       chainType: chainType.toString(),
       address,
       usdtBalance: usdtBalance.formatted,
+      usdtRawBalance: usdtBalance.raw.toString(),
+      usdtDecimals: usdtBalance.decimals,
       nativeBalance: nativeBalance.formatted,
       nativeSymbol: config.nativeSymbol,
     };

backend/services/blockchain-service/src/infrastructure/blockchain/evm-provider.adapter.ts

@@ -10,7 +10,10 @@ const ERC20_TRANSFER_EVENT_ABI = [
 ];
 
 // ERC20 balanceOf ABI
-const ERC20_BALANCE_ABI = ['function balanceOf(address owner) view returns (uint256)'];
+const ERC20_BALANCE_ABI = [
+  'function balanceOf(address owner) view returns (uint256)',
+  'function decimals() view returns (uint8)',
+];
 
 export interface TransferEvent {
   txHash: string;
@@ -127,8 +130,11 @@ export class EvmProviderAdapter {
   ): Promise<TokenAmount> {
     const provider = this.getProvider(chainType);
     const contract = new Contract(tokenContract, ERC20_BALANCE_ABI, provider);
-    const balance = await contract.balanceOf(address);
-    return TokenAmount.fromRaw(balance, 18);
+    const [balance, decimals] = await Promise.all([
+      contract.balanceOf(address),
+      contract.decimals(),
+    ]);
+    return TokenAmount.fromRaw(balance, Number(decimals));
   }
 
   /**