Feature request: PGP verification for email signatures

**Is your feature request related to a problem? Please describe.**
Wanted to verify some PGP mail signatures, but couldn't get it working.
I just exported the mail from Thunderbird and copy pasted the content into the input field, but then it either says: _Couldn't verify message: Error: Cannot verify detached signature without data input_ or if I manually trim it down to the OpenPGP section and add _-----BEGIN PGP SIGNED MESSAGE-----_, it tells me _Couldn't verify message: Error: Sig verify error: missing ASN header for MD5_.

**Describe the solution you'd like**
Would be nice if one could copy paste a PGP signed mail directly into the input field without converting or something.

**Describe alternatives you've considered**
I have tried to verify the signature with Kleopatra and Gpg4win CLI, but also without success. Seems like I am missing something here.

Start by reproducing the reported input-field behavior with a PGP-signed email exported from Thunderbird, including the full message and the OpenPGP section. Trace the handling of detached signatures and the reported ASN-header error; done means a directly pasted signed email can be verified without manual conversion.

Written by the indexing model from the issue text.