[Story] EX multi-account: deep-link sign-in resolves to the right account

### Description

* As a user opening a sign-in deep link that specifies a server/domain (and optionally a username)
* I want to be taken to the right place — my existing account, a fresh sign-in, or the add-account flow — based on whether I'm already signed in
* So that deep-linked sign-in works correctly now that multiple accounts are possible

# Acceptance criteria
- Given the deep link includes a username, when the user has no accounts signed in yet, then behavior is unchanged from today.
- Given the deep link includes a username, when the user already has that specific account signed in, then the app opens and switches the active account to that one.
- Given the deep link includes a username, when the user has other accounts but not that specific one, then the user is taken into the "Add account" flow.
- Given the deep link does not include a username, when the user has no accounts signed in yet, then behavior is unchanged from today.
- Given the deep link does not include a username, when the user already has one or more accounts signed in, then the "Add account" flow is triggered with the specified homeserver pre-filled.
- Given the deep link's `login_hint` doesn't cleanly match a signed-in account (e.g. it's an email address rather than a matrix ID), then proceed as "not logged in to this account" (e.g. into the add-account flow).

# Dependencies
- None

# Out of scope
- Matching of arbitrary `login_hint` values (e.g. email addresses) to a specific signed-in account

# Questions
- [x] None

# Subtasks

### Android
-

### iOS
-

### Rust
-

### Other
-

Start by locating the deep-link sign-in handling in the Android, iOS, and Rust client repositories referenced by the subtasks. Compare the existing behavior with each acceptance-criteria scenario, then verify that account switching, unchanged first-sign-in behavior, and the pre-filled Add account flow work for the specified homeserver.

Written by the indexing model from the issue text.