qr-code-dialog: Add .monospace style class in the "Password" row

(not sure if this needs approval from the design team)

This improves the readability of the password characters, because the password might still need to be used if scanning the QR code fails.

Before	After

The .monospace doesn't affect the title of property rows anymore in libadwaita 1.6 (libadwaita!1084 (merged))