Fixes: #2257.
If a user had selected a watch-only address in the wallet, the backend `walletModel.currentAccount` property would also change. When loading a transaction modal, this property was used to set the `selectedAccount` value for the “from” `AccountSelector`, regardless if the account was a watch-only address.
This PR updates the logic for `selectedAccount` such that it will only allow the account to be selected if it is not a watch-only account.