Allow users to create an optional login alias that is not an email address and is used only for authentication.

Example:

• Account email: "username@protonmail.com"
• Login alias: "shaolintemple" (as an example)

User can log in with either:

• email address
• existing username (email without domain)
• login alias
• whichever they prefer

The login alias would:

• Not receive email
• Not resolve to @proton.me / @protonmail.com
• Be usable only on the login screen
• Be fully optional (opt-in)

Why:

For power users, this adds defense-in-depth by preventing easy inference of a user’s email address from observed login behavior (shoulder surfing, screen sharing, key-logging, etc) without breaking existing workflows or usability.