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

test/unit-tests/components/viewmodels/settings/encryption/KeyStoragePanelViewModel-test.ts

@@ -87,5 +87,8 @@ describe("KeyStoragePanelViewModel", () => {
         await result.current.setEnabled(false);
 
         expect(mocked(matrixClient.getCrypto()!.disableKeyStorage)).toHaveBeenCalled();
+        expect(mocked(matrixClient.setAccountData)).toHaveBeenCalledWith("m.org.matrix.custom.backup_disabled", {
+            disabled: true,
+        });
     });
 });