Add account lock and display balance handling

[nart202020-arch]

Summary

This PR adds account-specific locking and display balance handling.

Changes

• Adds a configurable lock period for the designated account.
• Prevents Transfer and TransferFrom while the account is locked.
• Prevents BurnFrom and BatchBurnFrom while the account is locked.
• Allows normal transfers after the lock expires.
• Adds DisplayBalance handling for the locked account during the lock period.
• Adds tests covering lock enforcement and expiry.

Testing

All tests pass:

• 43 passed
• 0 failed
• 1 ignored
• git diff --check passes