Proton webmail finally supports FIDO2/U2F hardware security keys.

Please add this support to the mobile apps as well. Yubikey has inexpensive NFC-enabled hardware security keys. NFC or not, Proton's app should also support this as an option.

Currently you can only log into mobile with an app-based TOTP code or a recovery code, and you can only unlock a logged in session with a PIN or biometrics.

It should be possible to make the only option a hardware security key for all Proton apps.