fix: use cryptography library signature directly (no .signature attribute)

- cryptography library's Ed25519PrivateKey.sign() returns just signature bytes
- Not a SignedMessage object like PyNaCl
- Reverted .signature attribute access
- Fixes AttributeError when signing transactions

cli/aitbc_cli/commands/transactions.py

@@ -116,8 +116,7 @@ def _send_transaction_impl(from_wallet: str, to_address: str, amount: float, fee
     
     # Sign transaction payload
     message = json.dumps(transaction_payload, sort_keys=True).encode()
-    signed_message = private_key.sign(message)
-    signature = signed_message.signature  # Extract 64-byte signature only
+    signature = private_key.sign(message)  # cryptography library returns just signature bytes
     
     # Submit to blockchain with nested structure
     transaction = {