Enforce Secure Backup completion when requested by HS

This removes all buttons to escape the Secure Backup setup flow when the
matching `.well-known` setting is set by homeserver.

Part of https://github.com/vector-im/element-web/issues/14954

src/CrossSigningManager.js

@@ -21,6 +21,7 @@ import { deriveKey } from 'matrix-js-sdk/src/crypto/key_passphrase';
 import { decodeRecoveryKey } from 'matrix-js-sdk/src/crypto/recoverykey';
 import { _t } from './languageHandler';
 import {encodeBase64} from "matrix-js-sdk/src/crypto/olmlib";
+import { isSecureBackupRequired } from './utils/WellKnownUtils';
 
 // This stores the secret storage private keys in memory for the JS SDK. This is
 // only meant to act as a cache to avoid prompting the user multiple times
@@ -208,7 +209,18 @@ export async function accessSecretStorage(func = async () => { }, forceReset = f
                 {
                     force: forceReset,
                 },
-                null, /* priority = */ false, /* static = */ true,
+                null,
+                /* priority = */ false,
+                /* static = */ true,
+                /* options = */ {
+                    onBeforeClose(reason) {
+                        // If Secure Backup is required, you cannot leave the modal.
+                        if (reason === "backgroundClick") {
+                            return !isSecureBackupRequired();
+                        }
+                        return true;
+                    },
+                },
             );
             const [confirmed] = await finished;
             if (!confirmed) {