Refactor wallet balance handling and integrate PaymentService for centralized management

- Replaced direct wallet balance computation in Navbar and WalletPage with a centralized totalBalance property from PaymentService, improving code maintainability.
- Updated CreateProductDialog, CreateStoreDialog, and MerchantStore components to utilize PaymentService for retrieving wallet admin and invoice keys, enhancing consistency across the application.
- These changes streamline wallet management and improve the overall architecture of the wallet module.

src/core/services/PaymentService.ts

@@ -59,6 +59,15 @@ export class PaymentService extends BaseService {
     return this.authService?.user?.value?.wallets || []
   }
 
+  /**
+   * Get total balance across all user wallets in millisatoshis
+   */
+  get totalBalance(): number {
+    return this.userWallets.reduce((total: number, wallet: any) => {
+      return total + (wallet.balance_msat || 0)
+    }, 0)
+  }
+
   /**
    * Check if user has any wallet with balance
    */