wallet: add BIP39 passphrase support throughout HD lifecycle

hdPassphrase was hardcoded to empty string in DeriveHDKey, meaning users
who set a BIP39 passphrase during seed creation would derive different
addresses after restoration. This adds proper passphrase storage,
encryption, and decryption alongside the existing mnemonic handling:

- wallet.h: hdPassphrase + vchCryptedHDPassphrase + hdPassphraseIV fields
- wallet.cpp: Lock/Unlock/EncryptWallet/SetHDSeed all handle passphrase
  with the same encrypt/decrypt lifecycle as the mnemonic
- DeriveHDKey now passes hdPassphrase to DeriveTriangles (not hardcoded )
- walletdb.h: WriteHDPassphrase/WriteHDCryptedPassphrase/EraseHDPassphrase
- walletdb.cpp: ReadKeyValue handles hdpassphrase/hdcpassphrase records
- rpcwallet.cpp: hdnew/hdshow show passphrase_used + warnings

Also: i2p.cpp hardens I2P private key file permissions to owner-only.

From Claude's uncommitted work on E:\repos\triangles (SAMI-PC). The rest
of Claude's modernization (Boost removal, RPC rewrite, RocksDB default,
SQLite wallet) was already committed to master in bfdb399 and follow-ups.

src/i2p.cpp

@@ -223,6 +223,18 @@ bool CI2PSession::LoadOrCreateDestination(std::string& strPrivKeyRet)
                 if (out.is_open()) {
                     out << priv << std::endl;
                     out.close();
+                    // The I2P destination private key identifies this node on
+                    // the I2P network: owner-only permissions, like Tor's
+                    // hidden-service secret key. (No-op semantics differ on
+                    // Windows ACLs; harmless there.)
+                    std::error_code ec;
+                    std::filesystem::permissions(keyPath,
+                        std::filesystem::perms::owner_read |
+                        std::filesystem::perms::owner_write,
+                        std::filesystem::perm_options::replace, ec);
+                    if (ec)
+                        printf("I2P: WARNING could not restrict permissions on %s: %s\n",
+                               keyPath.string().c_str(), ec.message().c_str());
                     printf("I2P: generated and saved new persistent destination\n");
                     ok = true;
                 } else {