feat: add offer code redemption support for iOS and Android (#90)

## 🎟 Offer Code Redemption (Closes #87)

Adds offer code redemption support for both platforms.

### ✅ Changes

* **iOS**: Exposes `presentCodeRedemptionSheet` (works on real devices
only)
* **Android**: Adds `openRedeemOfferCodeAndroid` to open Play Store
redeem page
* **Example**: New screen showing usage on both platforms
* **Docs**: Added guide with usage and testing instructions

### 🧪 Testing

* iOS: Test with promo codes via TestFlight or App Store
* Android: Use promo codes from Google Play Console

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyochan

.claude/settings.local.json

@@ -58,3 +58,6 @@ yarn-error.log
 
 # Expo
 .expo/*
+
+# Claude settings
+.claude/settings.local.json

.gitignore

@@ -0,0 +1,143 @@
+---
+title: Offer Code Redemption
+sidebar_label: Offer Code Redemption
+---
+
+# Offer Code Redemption
+
+This guide explains how to implement offer code redemption functionality in your app using expo-iap.
+
+## Overview
+
+Offer codes (also known as promo codes or redemption codes) allow users to redeem special offers for in-app purchases and subscriptions. The implementation differs between iOS and Android platforms.
+
+## iOS Implementation
+
+On iOS, expo-iap provides a native method to present Apple's code redemption sheet directly within your app.
+
+### Usage
+
+```typescript
+import { presentCodeRedemptionSheet } from 'expo-iap';
+
+// Present the code redemption sheet
+try {
+  const result = await presentCodeRedemptionSheet();
+  if (result) {
+    console.log('Code redemption sheet presented successfully');
+    // The system will handle the redemption process
+    // Listen for purchase updates via purchaseUpdatedListener
+  }
+} catch (error) {
+  console.error('Failed to present code redemption sheet:', error);
+}
+```
+
+### Important Notes
+
+- This method only works on real iOS devices (not simulators)
+- Not available on tvOS
+- The redemption sheet is handled by the iOS system
+- After successful redemption, purchase updates will be delivered through your existing `purchaseUpdatedListener`
+
+## Android Implementation
+
+Google Play does not provide a direct API to redeem codes within the app. Instead, users must redeem codes through the Google Play Store app or website.
+
+### Usage
+
+```typescript
+import { openRedeemOfferCodeAndroid } from 'expo-iap';
+
+// Open Google Play Store redemption page
+try {
+  await openRedeemOfferCodeAndroid();
+  // This will open the Play Store where users can enter their codes
+} catch (error) {
+  console.error('Failed to open Play Store:', error);
+}
+```
+
+### Alternative Approach
+
+You can also direct users to redeem codes via a custom deep link:
+
+```typescript
+import { Linking } from 'react-native';
+
+const redeemCode = async (code: string) => {
+  const url = `https://play.google.com/redeem?code=${code}`;
+  await Linking.openURL(url);
+};
+```
+
+## Complete Example
+
+Here's a complete example that handles both platforms:
+
+```typescript
+import { Platform } from 'react-native';
+import { 
+  presentCodeRedemptionSheet,
+  openRedeemOfferCodeAndroid,
+  purchaseUpdatedListener 
+} from 'expo-iap';
+
+const handleRedeemCode = async () => {
+  try {
+    if (Platform.OS === 'ios') {
+      // Present native iOS redemption sheet
+      const result = await presentCodeRedemptionSheet();
+      if (result) {
+        console.log('Redemption sheet presented');
+      }
+    } else if (Platform.OS === 'android') {
+      // Open Play Store for Android
+      await openRedeemOfferCodeAndroid();
+    }
+  } catch (error) {
+    console.error('Error redeeming code:', error);
+  }
+};
+
+// Set up listener for purchase updates after redemption
+useEffect(() => {
+  const subscription = purchaseUpdatedListener((purchase) => {
+    console.log('Purchase updated after redemption:', purchase);
+    // Handle the new purchase/subscription
+  });
+
+  return () => {
+    subscription.remove();
+  };
+}, []);
+```
+
+## Best Practices
+
+1. **User Experience**: Clearly communicate to users where they can find and how to use offer codes
+2. **Error Handling**: Always wrap redemption calls in try-catch blocks
+3. **Platform Detection**: Use platform-specific methods appropriately
+4. **Purchase Validation**: Always validate purchases on your server after redemption
+
+## Testing
+
+### iOS Testing
+- Offer codes can only be tested on real devices
+- Use TestFlight or App Store Connect to generate test codes
+- Sandbox environment supports offer code testing
+
+### Android Testing
+- Test with promo codes generated in Google Play Console
+- Ensure your app is properly configured for in-app purchases
+
+## Troubleshooting
+
+### iOS Issues
+- **"Not available on simulator"**: Use a real device for testing
+- **Sheet doesn't appear**: Ensure StoreKit is properly configured
+- **User cancellation**: This is normal behavior and doesn't throw an error
+
+### Android Issues
+- **Play Store doesn't open**: Check if Play Store is installed and updated
+- **Invalid code**: Verify the code format and validity in Play Console

docs/docs/guides/offer-code-redemption.md

@@ -35,6 +35,7 @@ const sidebars: SidebarsConfig = {
       items: [
         'guides/purchases',
         'guides/lifecycle',
+        'guides/offer-code-redemption',
         'guides/troubleshooting',
         'guides/faq',
         'guides/migration',