Announce multi-account support

Show a one-time bottom sheet on the home screen to announce that several
accounts can now be added (element-hq/element-meta#3349).

- Add `Announcement.MultiAccount`, which is never shown by default.
- AppMigration11 queues the announcement on application upgrade only,
  and marks it as shown if the user already has several accounts.
- HomePresenter shows the bottom sheet only when the announcement is
  pending, the MultiAccount feature flag is enabled and the user has a
  single account.
- The "Add account" button hides the sheet, marks the announcement as
  dismissed and starts the add account flow through the new
  `HomeEntryPoint.Callback.navigateToAddAccount()`.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

features/announcement/api/src/main/kotlin/io/element/android/features/announcement/api/Announcement.kt

@@ -17,4 +17,6 @@ sealed interface Announcement {
     }
 
     data object NewNotificationSound : Announcement
+
+    data object MultiAccount : Announcement
 }