Support Standard SmartCard Authentication methods
Most SSH clients support Smartcard authentication where the private key is stored on smart cards (or yubi-keys and other similar HSM modules) and can be exposed via the industry standard PKSC#11 interfaces
please consider generic support of PKSC#11 for the desktop app, and potentially for mobile via NFC/USB interaction as mentioned in other ideas like
https://support.termius.com/forums/243649-ios-app/suggestions/34881820-nfc-support
