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/structures/_ToastContainer.pcss

@@ -79,6 +79,11 @@ Please see LICENSE files in the repository root for full details.
                 background-color: $primary-content;
             }
 
+            &.mx_Toast_icon_key_storage::after {
+                mask-image: url("@vector-im/compound-design-tokens/icons/settings-solid.svg");
+                background-color: $primary-content;
+            }
+
             &.mx_Toast_icon_labs::after {
                 mask-image: url("$(res)/img/element-icons/flask.svg");
                 background-color: $secondary-content;