[Story] EX multi-account: resolve the right account when opening a room/message permalink

### Description

* As a user with multiple signed-in accounts
* I want opening a room or message permalink to use the right account automatically where possible
* So that I don't have to guess which account has access, or manually switch first

# Acceptance criteria
- Given a permalink is opened and exactly one signed-in account is a member of that room, when the link is opened, then the app uses that account automatically.
- Given a permalink is opened and none of the signed-in accounts are members of that room, when the link is opened, then the user is prompted to choose which account to use, defaulting to the currently active account.
- Given a permalink is opened and more than one signed-in account is a member of that room, when the link is opened, then the app uses whichever of those accounts was most recently active.

# Dependencies
- None

# Out of scope
- Nothing

# Questions
- [x] None

# Subtasks

### Android
-

### iOS
-

### Rust
-

### Other
-

The issue names no implementation files, tests, or entry points; begin by locating permalink handling in the Android, iOS, and Rust client repositories. Done means the three account-selection acceptance criteria are implemented and verified in the affected clients.

Written by the indexing model from the issue text.