Shopify Store Credit Integration

How store credit syncs with Shopify Store Credit Accounts.

• Store Credit Accounts: Uses Shopify's native Store Credit Account system (GraphQL API)
• Account Creation: Accounts are created automatically when customer receives first credit
• Balance Sync: Credits and debits are synced in real-time to Shopify
• Currency Support: Supports multiple currencies based on Shopify store currency settings
• Account ID Storage: shopifyStoreCreditAccountId is stored locally for efficient updates
• Graceful Fallback: System works without Shopify - sync is optional enhancement

GraphQL Mutations: System uses storeCreditAccountCredit (to add credit) and storeCreditAccountDebit (to deduct credit) mutations. Account ID is retrieved from customer's storeCreditAccounts query. If account doesn't exist, ensureStoreCreditAccount creates it automatically.