Add methods to PaymentService for preferred wallet selection

- Introduced getPreferredWallet() to consistently retrieve the first wallet from userWallets.
- Added getPreferredWalletAdminKey() to obtain the admin key of the preferred wallet for administrative tasks.

These enhancements improve wallet management and ensure consistent wallet selection across the application.

src/core/services/PaymentService.ts

@@ -82,6 +82,24 @@ export class PaymentService extends BaseService {
     return wallets.find((wallet: any) => wallet.balance_msat > 0)
   }
 
+  /**
+   * Get the preferred wallet for operations (always use wallets[0] for consistency)
+   * This ensures consistent wallet selection across all modules
+   */
+  getPreferredWallet(): any | null {
+    const wallets = this.userWallets
+    if (!wallets.length) return null
+    return wallets[0]
+  }
+
+  /**
+   * Get the preferred wallet's admin key for administrative operations
+   */
+  getPreferredWalletAdminKey(): string | null {
+    const wallet = this.getPreferredWallet()
+    return wallet?.adminkey || null
+  }
+
   /**
    * Generate QR code for Lightning payment request
    */