UI: Import of a PSBT with a locked UTXO possible

If a user creates a transaction, saves it as Base64 encoded PSBT, lockes the UTXO involved in the meantime and then imports the transaction there is currently no error. Should be implemented here:

https://github.com/cryptoadvance/specter-desktop/blob/ef3483cc3374fbc8fd49f408b085db10f6411641/src/cryptoadvance/specter/wallet.py#L2006

Start in src/cryptoadvance/specter/wallet.py around line 2006 and trace the import path for Base64-encoded PSBTs. Reproduce the sequence of creating a transaction, locking its UTXO, and importing the saved PSBT. Done means the import reports an error instead of accepting a PSBT that uses a locked UTXO.

Written by the indexing model from the issue text.