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>

package.json

@@ -194,6 +194,7 @@
     "expo-paste-input": "^0.2.1",
     "expo-privacy-sensitive": "^0.2.0",
     "expo-screen-orientation": "~9.0.8",
+    "expo-secure-store": "~15.0.8",
     "expo-sharing": "~14.0.8",
     "expo-sms": "^14.0.7",
     "expo-splash-screen": "~31.0.13",