fix: extract 64-byte signature from SignedMessage using .signature attribute

- private_key.sign() returns SignedMessage (signature + message)
- Extract only the 64-byte signature using .signature attribute
- Fixes signature length mismatch (was sending 216 hex chars instead of 128)
- RPC expects only the 64-byte signature portion

cli/aitbc_cli/commands/transactions.py

@@ -116,7 +116,8 @@ 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()
-    signature = private_key.sign(message)
+    signed_message = private_key.sign(message)
+    signature = signed_message.signature  # Extract 64-byte signature only
     
     # Submit to blockchain with nested structure
     transaction = {