Implement unified account authentication across Proton's Android apps, similar to Google and Yandex ecosystems. Once a user signs in to one Proton app, that account should be available for other Proton apps on the same device without requiring repeated manual login.

Description:
Currently, Proton apps on Android require separate sign-in for each app, even when the user is logging into the same Proton account. This creates unnecessary friction. If a user is already authenticated in Proton Mail, apps like Proton VPN or Proton Pass should be able to detect and reuse that existing account session instead of asking for login credentials again.