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.

feature-account-impl/src/main/java/io/novafoundation/nova/feature_account_impl/data/signer/secrets/SecretsSigner.kt

@@ -146,7 +146,8 @@ class SecretsSigner(
         return secretStoreV2.getKeypair(
             metaAccount = metaAccount,
             accountId = accountId,
-            isEthereumBased = multiChainEncryption is MultiChainEncryption.Ethereum
+            isEthereumBased = multiChainEncryption is MultiChainEncryption.Ethereum,
+            isBitcoinBased = metaAccount.bitcoinAddress?.contentEquals(accountId) == true
         )
     }
 
@@ -159,11 +160,12 @@ class SecretsSigner(
     private suspend fun SecretStoreV2.getKeypair(
         metaAccount: MetaAccount,
         accountId: AccountId,
-        isEthereumBased: Boolean
+        isEthereumBased: Boolean,
+        isBitcoinBased: Boolean = false,
     ) = if (hasChainSecrets(metaAccount.id, accountId)) {
         getChainAccountKeypair(metaAccount.id, accountId)
     } else {
-        getMetaAccountKeypair(metaAccount.id, isEthereumBased)
+        getMetaAccountKeypair(metaAccount.id, isEthereumBased, isBitcoinBased)
     }
 
     /**
@@ -173,6 +175,7 @@ class SecretsSigner(
         return when {
             substrateAccountId.contentEquals(accountId) -> substrateCryptoType?.let(MultiChainEncryption.Companion::substrateFrom)
             ethereumAccountId().contentEquals(accountId) -> MultiChainEncryption.Ethereum
+            bitcoinAddress?.contentEquals(accountId) == true -> MultiChainEncryption.Ethereum
             else -> {
                 val chainAccount = chainAccounts.values.firstOrNull { it.accountId.contentEquals(accountId) } ?: return null
                 val cryptoType = chainAccount.cryptoType ?: return null