v1.6: fix HIGH security vuln — remove plaintext cookie backup

- Remove KEY_COOKIES_BACKUP plaintext fallback from PreferencesManager
- getCookies() now fails closed (force re-login) if EncryptedSharedPreferences unavailable
- Set android:allowBackup="false" to prevent adb backup extraction of session data
- Add missing gradle-wrapper.jar to repo

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/src/main/AndroidManifest.xml

@@ -5,7 +5,7 @@
     <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
 
     <application
-        android:allowBackup="true"
+        android:allowBackup="false"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
         android:roundIcon="@mipmap/ic_launcher_round"