add expo-secure-store dependency and jest mocks

Adds expo-secure-store (~15.0.8, the SDK 54 bundled version) plus its
config plugin, which excludes the SecureStore keystore from Android
auto backup.

Also adds two global jest mocks: expo-secure-store, whose real entry
reads accessibility constants off the native module at import time, and
react-native-mmkv, since #/storage constructs an MMKV instance at module
scope. File-local mocks continue to take precedence.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app.config.js

@@ -240,6 +240,7 @@ module.exports = function (_config) {
       plugins: [
         'expo-video',
         'expo-localization',
+        'expo-secure-store',
         'expo-web-browser',
         [
           'react-native-edge-to-edge',