feat: Bitcoin key derivation, secrets storage, and chain-family plumbing

Derives a BIP84 (native SegWit) keypair alongside the existing substrate/
ethereum/tron ones, stores it in MetaAccountSecrets/CloudBackup, persists
bitcoinPublicKey/bitcoinAddress on meta_accounts (DB migration 74->75), and
threads isBitcoinBased through Chain/ChainLocal/mappers/ChainExt/ChainRegistry
the same way isTronBased was wired. Adds BitcoinAddressBackfillMigration,
mirroring TronAddressBackfillMigration, so existing wallets get a Bitcoin
address without re-import.

feature-settings-impl/src/main/java/io/novafoundation/nova/feature_settings_impl/domain/utils/CustomChainFactory.kt

@@ -125,6 +125,7 @@ class CustomChainFactory(
             types = prefilledChain?.types,
             isEthereumBased = isEthereumBased,
             isTronBased = false,
+            isBitcoinBased = false,
             isTestNet = prefilledChain?.isTestNet.orFalse(),
             source = Chain.Source.CUSTOM,
             hasSubstrateRuntime = hasSubstrateRuntime,