Key storage out of sync: reset key backup when needed (#31279)

* add function to pause device listener

* add function to check if key backup key missing both locally and in 4s

* reset backup if backup key missing both locally and in 4s

* fixup! add function to check if key backup key missing both locally and in 4s

* Drop KEY_STORAGE_OUT_OF_SYNC_STORE in favour of checking cross-signing

Check if cross-signing needs resetting, because that seems to be what
KEY_STORAGE_OUT_OF_SYNC_STORE is actually trying to do.

* add a function for resetting key backup and waiting until it's ready

* trigger key storage out of sync toast when missing backup key locally

and fetch it when user enters their recovery key

* reset backup when needed if user forgets recovery key

* rename function as suggested in code review

src/utils/crypto/resetKeyBackup.ts

@@ -0,0 +1,20 @@
+/*
+Copyright 2025 Element Creations 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.
+*/
+
+import { type CryptoApi } from "matrix-js-sdk/src/crypto-api";
+
+/**
+ * Creates a new key backup version, and wait until it is enabled.
+ *
+ * This is typically used within a {@link DeviceListener.pause()} call, to
+ * ensure that the device listener doesn't check the backup status until after the
+ * key backup is active.
+ */
+export async function resetKeyBackupAndWait(crypto: CryptoApi): Promise<void> {
+    await crypto.resetKeyBackup();
+    await crypto.checkKeyBackupAndEnable();
+}