This is why I send the public key as a attachment in every mail. This way you just import it.
On https://account.proton.me/u/0/mail/encryption-keys you can also import keys for advanced users like you.
Proton mail doesn't want to be able to read your mails so your browser needs to unlock your mailbox. IMAP would require them to be able to read your mailbox so they provide you with the bridge so you can install IMAP on a host YOU trust.
@dgr @fidel I understand why the proton bridge is needed for imap access to proton, but I don't think it's right architectural decision. migadu's approach looks more apropriate to me:
https://migadu.com/procon/#not-encrypted