[Story] EX multi-account: push notifications reflect and switch the active account

### Description

* As a user with multiple signed-in accounts
* I want notifications to take me to the right room and account, and to be able to tell accounts apart when it matters
* So that I land in the correct conversation and can distinguish duplicate-looking notifications across accounts

# Acceptance criteria
- Given a notification arrives for a room that the user has joined with only one of their signed-in accounts, then it does not show account information by (room name/avatar is normally sufficient to disambiguate).
- Given a notification arrives for a room that the user has joined with more than one of their signed-in accounts, then the notification shows account information (e.g. MXID).
- Given a user has joined a room with more than one of their signed-in accounts, then a notifications for such rooms are not de-duplicated and shown for each of the accounts.
- Given the user sends a message themselves from account A into a room also joined by account B, when the message is sent, then account B does not show a notification for the user's own message.
- Given the user taps a notification, when the app opens, then it navigates to the corresponding room and switches the active account to the one the notification belongs to.

# Dependencies
- None

# Out of scope
- General de-duplication of notifications across accounts sharing a room (not planned; keeping them separate is what gives the user the choice of which account to open)

# Questions
- [ ] Can the account-info-on-notification behavior be limited to only rooms that exist in more than one of the user's accounts, to reduce visual noise, rather than being shown more broadly? Needs design confirmation.

# Subtasks

### Android
-

### iOS
-

### Rust
-

### Other
-

Start with the acceptance criteria and the Android, iOS, and Rust subtask sections; no implementation files or tests are identified in this story. Break the work into platform-specific issues, resolve the open account-information design question, and consider the story done when each listed notification and account-switching behavior is implemented and verified across the relevant clients.

Written by the indexing model from the issue text.