[Story] EX multi-account: "Manage account and devices" opens the correct (active) account in MAS

### Description

* As a user with multiple signed-in accounts
* I want "Manage account and devices" to open Matrix Authentication Service (MAS) for the account I actually have active
* So that I manage the account I intended to, not a different one

# Acceptance criteria
- Given account A is active, when the user taps "Manage account and devices", then MAS opens scoped to account A. This is regardless if account A and user's other accounts are on the same or different homeservers.

# Dependencies
- None

# Out of scope
- Nothing

# Questions
- [x] None

# Subtasks

### Android
-

### iOS
-

### Rust
-

### Other
-

Start by reviewing the acceptance criterion for the active account and the Android, iOS, and Rust subtask sections. Identify which client platform repository owns “Manage account and devices” and how it opens MAS, then define completion as MAS opening scoped to the active account across same- and different-homeserver cases.

Written by the indexing model from the issue text.