DAR-105 "Feature/ thirdweb erc4337 smart wallet"
The original scaffold (DAR-104) used ThirdWeb Engine's legacy POST /backend-wallet/create
endpoint, which creates an EOA backend wallet — not a user-owned smart account.
This MR replaces that with the correct ERC-4337 Account Abstraction flow:
each investor gets a deterministic smart contract wallet (counterfactual address) tied
to their userId, signed by a shared admin backend wallet.
Closes DAR-105