Choose an Authentication Approach

Background: Native [platform] app, [SwiftUI+Combine / Compose]. Users
need to [sign up / sign in / stay signed in across launches]. Sensitive
data involved: [yes/no—what]. Backend/auth provider: [chosen one, or
"undecided"].

Use case: Design the authentication approach and the sign-in options to
offer.

Implementation details:
- Compare email/password, magic link, OAuth social logins, and passkeys
- iOS: when Sign in with Apple is required by App Store rules, plus Face
  ID / Touch ID for re-auth
- Android: Credential Manager / Sign in with Google, and biometric
  prompt for re-auth
- Token storage in the secure store (Keychain / Keystore), refresh
  handling, and what happens on token expiry mid-session
- Sign-out and account-deletion (store-required) flows

Layer & dependencies: Auth state should be a single observable source
the whole app reads; tokens never touch plain UserDefaults/SharedPrefs.

Deliverable: A recommended auth design, the sign-in options with their
tradeoffs and store-compliance notes, the token lifecycle (login →
refresh → expiry → logout), and where each piece lives in my layers.