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>

appnav/src/main/kotlin/io/element/android/appnav/LoggedInFlowNode.kt

@@ -359,6 +359,10 @@ class LoggedInFlowNode(
                     override fun navigateToBugReport() {
                         callback.navigateToBugReport()
                     }
+
+                    override fun navigateToAddAccount() {
+                        callback.navigateToAddAccount()
+                    }
                 }
                 homeEntryPoint.createNode(
                     parentNode = this,