backend: add litecoin to btcdirect supported coins

BTC Direct supports buying and selling LTC through their private
trading desk, so the link and info should also be shown to Litecoin
accounts on exchange selection.

backend/exchanges/btcdirect.go

@@ -34,7 +34,7 @@ func GetBtcDirectSupportedRegions() []string {
 // IsBtcDirectSupported is true if coin.Code and region are supported by BtcDirect.
 func IsBtcDirectSupported(coinCode coin.Code, region string) bool {
 	supportedCoins := []coin.Code{
-		coin.CodeBTC, coin.CodeTBTC, coin.CodeETH, coin.CodeSEPETH,
+		coin.CodeBTC, coin.CodeTBTC, coin.CodeLTC, coin.CodeTLTC, coin.CodeETH, coin.CodeSEPETH,
 		"eth-erc20-usdt", "eth-erc20-usdc", "eth-erc20-link"}
 
 	coinSupported := slices.Contains(supportedCoins, coinCode)