feat(wallet): show pending mempool balance and exclude unconfirmed UTXOs from spending

TransactionFactory now selects only confirmed UTXOs (height > 0), so
mempool funds are visible as pending balance but never spendable, with
a clear error when only unconfirmed funds are available. GUI and CLI
labels updated to "in attesa di conferma (non spendibile)".

tests/PalladiumWallet.Tests/Wallet/TransactionFactoryTests.cs

@@ -91,6 +91,19 @@ public class TransactionFactoryTests
             feeRateSatPerVByte: 2, changeIndex: 0));
     }
 
+    [Fact]
+    public void Gli_utxo_in_mempool_non_sono_spendibili()
+    {
+        var account = Account();
+        var (utxos, txs) = Fund(account, 1_000_000);
+        utxos[0].Height = 0; // in mempool: visibile nel saldo pending, non spendibile
+
+        var ex = Assert.Throws<WalletSpendException>(() => new TransactionFactory(account).Build(
+            utxos, txs, account.GetReceiveAddress(1), amountSats: 100_000,
+            feeRateSatPerVByte: 2, changeIndex: 0));
+        Assert.Contains("in attesa di conferma", ex.Message);
+    }
+
     [Fact]
     public void Gli_utxo_congelati_sono_esclusi_dalla_spesa()
     {