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/migration/impl/src/main/kotlin/io/element/android/features/migration/impl/migrations/AppMigration11.kt

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.features.migration.impl.migrations
+
+import dev.zacsweers.metro.AppScope
+import dev.zacsweers.metro.ContributesIntoSet
+import io.element.android.features.announcement.api.Announcement
+import io.element.android.features.announcement.api.AnnouncementService
+import io.element.android.libraries.sessionstorage.api.SessionStore
+
+/**
+ * Ensure the multi-account announcement is displayed, but only on application upgrade,
+ * and only if the user does not already have several accounts.
+ */
+@ContributesIntoSet(AppScope::class)
+class AppMigration11(
+    private val sessionStore: SessionStore,
+    private val announcementService: AnnouncementService,
+) : AppMigration {
+    override val order: Int = 11
+
+    override suspend fun migrate(isFreshInstall: Boolean) {
+        if (isFreshInstall) return
+
+        if (sessionStore.getAllSessions().size > 1) {
+            // The user already uses several accounts, no need to announce the feature
+            announcementService.onAnnouncementDismissed(Announcement.MultiAccount)
+        } else {
+            announcementService.showAnnouncement(Announcement.MultiAccount)
+        }
+    }
+}

features/migration/impl/src/test/kotlin/io/element/android/features/migration/impl/migrations/AppMigration11Test.kt

@@ -0,0 +1,65 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.features.migration.impl.migrations
+
+import io.element.android.features.announcement.api.Announcement
+import io.element.android.features.rageshake.test.logs.FakeAnnouncementService
+import io.element.android.libraries.sessionstorage.test.InMemorySessionStore
+import io.element.android.libraries.sessionstorage.test.aSessionData
+import io.element.android.tests.testutils.lambda.lambdaRecorder
+import io.element.android.tests.testutils.lambda.value
+import kotlinx.coroutines.test.runTest
+import org.junit.Test
+
+class AppMigration11Test {
+    @Test
+    fun `migration on fresh install should not invoke the AnnouncementService`() = runTest {
+        val service = FakeAnnouncementService()
+        val migration = AppMigration11(
+            sessionStore = InMemorySessionStore(initialList = listOf(aSessionData())),
+            announcementService = service,
+        )
+        // FakeAnnouncementService lambdas throw by default
+        migration.migrate(isFreshInstall = true)
+    }
+
+    @Test
+    fun `migration on upgrade with a single account should show the announcement`() = runTest {
+        val showAnnouncementResult = lambdaRecorder<Announcement, Unit> { }
+        val service = FakeAnnouncementService(
+            showAnnouncementResult = showAnnouncementResult,
+        )
+        val migration = AppMigration11(
+            sessionStore = InMemorySessionStore(initialList = listOf(aSessionData())),
+            announcementService = service,
+        )
+        migration.migrate(isFreshInstall = false)
+        showAnnouncementResult.assertions().isCalledOnce()
+            .with(value(Announcement.MultiAccount))
+    }
+
+    @Test
+    fun `migration on upgrade with several accounts should mark the announcement as dismissed`() = runTest {
+        val onAnnouncementDismissedResult = lambdaRecorder<Announcement, Unit> { }
+        val service = FakeAnnouncementService(
+            onAnnouncementDismissedResult = onAnnouncementDismissedResult,
+        )
+        val migration = AppMigration11(
+            sessionStore = InMemorySessionStore(
+                initialList = listOf(
+                    aSessionData(sessionId = "@alice:server.org"),
+                    aSessionData(sessionId = "@bob:server.org"),
+                )
+            ),
+            announcementService = service,
+        )
+        migration.migrate(isFreshInstall = false)
+        onAnnouncementDismissedResult.assertions().isCalledOnce()
+            .with(value(Announcement.MultiAccount))
+    }
+}