Prompt the user when key storage is unexpectedly off (#29912)

* Assert that we set backup_disabled when turning off key storage

* Prompt the user when key storage is unexpectedly off

* Playwright tests for the Turn on key storage toast

res/css/views/dialogs/_ConfirmKeyStorageOffDialog.pcss

@@ -0,0 +1,16 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+.mx_ConfirmKeyStorageOffDialog {
+    .mx_Dialog_border {
+        width: 600px;
+    }
+
+    .mx_EncryptionCard {
+        text-align: center;
+    }
+}