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-api/src/main/java/io/novafoundation/nova/feature_account_api/data/model/Fee.kt

@@ -63,6 +63,18 @@ class SubstrateFee(
     override val asset: Chain.Asset
 ) : Fee
 
+/**
+ * Fee for a Bitcoin transaction, denominated in sats: `feeRate (sat/vB) * estimated vsize` - see
+ * `RealBitcoinTransactionService` for how both are derived. The network only ever collects what miners include
+ * in a block, which is exactly this amount (unlike account-model chains, a Bitcoin fee is not a cap/estimate
+ * that gets partially refunded - it is the literal difference between input and output values).
+ */
+class BitcoinFee(
+    override val amount: BigInteger,
+    override val submissionOrigin: SubmissionOrigin,
+    override val asset: Chain.Asset
+) : Fee
+
 class SubstrateFeeBase(
     override val amount: BigInteger,
     override val asset: Chain.Asset

feature-account-api/src/main/java/io/novafoundation/nova/feature_account_api/data/secrets/SecretStoreExt.kt

@@ -25,14 +25,14 @@ suspend fun SecretStoreV2.getAccountSecrets(
 
 fun AccountSecrets.keypair(chain: Chain): Keypair {
     return fold(
-        left = { mapMetaAccountSecretsToKeypair(it, ethereum = chain.isEthereumBased) },
+        left = { mapMetaAccountSecretsToKeypair(it, ethereum = chain.isEthereumBased, bitcoin = chain.isBitcoinBased) },
         right = { mapChainAccountSecretsToKeypair(it) }
     )
 }
 
 fun AccountSecrets.derivationPath(chain: Chain): String? {
     return fold(
-        left = { mapMetaAccountSecretsToDerivationPath(it, ethereum = chain.isEthereumBased) },
+        left = { mapMetaAccountSecretsToDerivationPath(it, ethereum = chain.isEthereumBased, bitcoin = chain.isBitcoinBased) },
         right = { it[ChainAccountSecrets.DerivationPath] }
     )
 }