[Story] EX multi-account: screen lock / PIN remains app-level

### Description

* As a user with multiple signed-in accounts and screen lock enabled
* I want one PIN to protect the whole app regardless of how many accounts are signed in
* So that my security experience doesn't get more complicated as I add accounts

# Acceptance criteria
- Given screen lock/PIN is enabled, when the user has multiple signed-in accounts, then the PIN still protects the app as a whole.
- Given the user enters an invalid PIN, when the existing failure/sign-out behavior is triggered, then all accounts are signed out.

# Dependencies
- None

# Out of scope
- Nothing

# Questions
- [x] None

# Subtasks

### Android
-

### iOS
-

### Rust
-

### Other
-

No implementation files, tests, or entry points are named. Start by identifying the Android, iOS, and Rust client work needed for the screen-lock flow; done means one PIN protects all signed-in accounts and an invalid PIN signs out every account.

Written by the indexing model from the issue text.