Can't install on Windows

Not able to install `@breeztech/breez-sdk-spark-react-native` on Windows

### Reproduction

Note: Use a windows machine

1. Init expo

```bash
npx create-expo-app@latest my-app --template blank-typescript
```

2. Try installing breez-sdk-spark-react-native

```bash
npx expo install @breeztech/breez-sdk-spark-react-native
```

Result: `Error: npm install --save @breeztech/breez-sdk-spark-react-native exited with non-zero code: 1`

### The Problem

The issue is that the package's postinstall script uses a Unix-style path `/scripts/postinstall.sh` which Windows `cmd.exe` can't execute.

### Possible fix

Move `postinstall` to a `js` script

Start by inspecting the package's postinstall configuration and the referenced scripts/postinstall.sh. Reproduce the installation with the stated Expo commands on Windows, then verify that the package installs successfully without cmd.exe failing to execute the postinstall step.

Written by the indexing model from the issue text.