- Add src/crypto.py with AES-256-CBC pw_encode/pw_decode
using double SHA256 key derivation (Electrum standard)
- Add encrypt_wallet() and decrypt_wallet() to src/hd_wallet.py
- Prompt for encryption password when saving HD wallet to file
- Add 13 encryption/decryption tests to tests/test_hd_wallet.py
- Add pycryptodome to requirements.txt