Add OS auth to local backup key viewer

ts/util/promptOSAuth.ts

@@ -0,0 +1,38 @@
+// Copyright 2025 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { ipcRenderer } from 'electron';
+import type {
+  PromptOSAuthReasonType,
+  PromptOSAuthResultType,
+} from './os/promptOSAuthMain';
+
+export async function promptOSAuth(
+  reason: PromptOSAuthReasonType
+): Promise<PromptOSAuthResultType> {
+  return new Promise<PromptOSAuthResultType>((resolve, _reject) => {
+    let localeString: string | undefined;
+
+    // TODO: DESKTOP-8895
+    if (window.Signal.OS.isMacOS()) {
+      if (reason === 'enable-backups') {
+        localeString = 'enable backups';
+      } else if (reason === 'view-aep') {
+        localeString = 'show your backup key';
+      }
+    }
+
+    if (window.Signal.OS.isWindows()) {
+      if (reason === 'enable-backups') {
+        localeString = 'Verify your identity to enable backups.';
+      } else if (reason === 'view-aep') {
+        localeString = 'Verify your identity to view your backup key.';
+      }
+    }
+
+    ipcRenderer.once(`prompt-os-auth:${reason}`, (_, response) => {
+      resolve(response ?? 'error');
+    });
+    ipcRenderer.send('prompt-os-auth', { reason, localeString });
+  });
+}