feat: Bitcoin transaction construction, signing, and sending

RealBitcoinTransactionService builds and signs native SegWit transactions
client-side (mempool.space only exposes UTXOs/fee-rate/broadcast, not
construction, unlike TronGrid): greedy largest-first UTXO selection over
confirmed UTXOs, inputs*68 + outputs*31 + 11 vsize heuristic for fees
(matching pezkuwi-exchange's proven approach), RBF-signaled inputs, and a
546-sat dust threshold that folds sub-dust change into the fee rather than
creating an uneconomical output.

Each input gets its own BIP143 sighash, signed via the existing raw-hash
signing primitive (NovaSigner.signRaw with skipMessageHashing) already used
for Ethereum/Tron, then DER-encoded with low-S normalization - no new
signing call path was added, only the DER encoding step.

Also fixes multiChainEncryptionFor/getMetaAccountKeypair to recognize
Bitcoin accounts: previously neither function had any Bitcoin (or Tron)
branch, so signing would have silently used the wrong keypair. Threads a
bitcoin flag through mapMetaAccountSecretsToKeypair/DerivationPath and
AccountSecrets.keypair(chain)/derivationPath(chain) the same way ethereum
already is.

common/src/main/java/io/novafoundation/nova/common/data/secrets/v2/SecretStoreV2.kt

@@ -156,17 +156,21 @@ val AccountSecrets.isChainAccountSecrets
 suspend fun SecretStoreV2.getMetaAccountKeypair(
     metaId: Long,
     isEthereum: Boolean,
+    isBitcoin: Boolean = false,
 ): Keypair = withContext(Dispatchers.Default) {
     val secrets = getMetaAccountSecrets(metaId) ?: noMetaSecrets(metaId)
 
-    mapMetaAccountSecretsToKeypair(secrets, isEthereum)
+    mapMetaAccountSecretsToKeypair(secrets, isEthereum, isBitcoin)
 }
 
 fun mapMetaAccountSecretsToKeypair(
     secrets: EncodableStruct<MetaAccountSecrets>,
     ethereum: Boolean,
+    bitcoin: Boolean = false,
 ): Keypair {
-    val keypairStruct = if (ethereum) {
+    val keypairStruct = if (bitcoin) {
+        secrets[MetaAccountSecrets.BitcoinKeypair] ?: noBitcoinSecret()
+    } else if (ethereum) {
         secrets[MetaAccountSecrets.EthereumKeypair] ?: noEthereumSecret()
     } else {
         secrets[MetaAccountSecrets.SubstrateKeypair]
@@ -178,8 +182,11 @@ fun mapMetaAccountSecretsToKeypair(
 fun mapMetaAccountSecretsToDerivationPath(
     secrets: EncodableStruct<MetaAccountSecrets>,
     ethereum: Boolean,
+    bitcoin: Boolean = false,
 ): String? {
-    return if (ethereum) {
+    return if (bitcoin) {
+        secrets[MetaAccountSecrets.BitcoinDerivationPath]
+    } else if (ethereum) {
         secrets[MetaAccountSecrets.EthereumDerivationPath]
     } else {
         secrets[MetaAccountSecrets.SubstrateDerivationPath]
@@ -198,6 +205,8 @@ private fun noChainSecrets(metaId: Long, accountId: ByteArray): Nothing {
 
 private fun noEthereumSecret(): Nothing = error("No ethereum keypair found")
 
+private fun noBitcoinSecret(): Nothing = error("No bitcoin keypair found")
+
 fun mapKeypairStructToKeypair(struct: EncodableStruct<KeyPairSchema>): Keypair {
     return Keypair(
         publicKey = struct[KeyPairSchema.PublicKey],