gui: 'getAvailableBalance', include watch only balance

Only for wallets with private keys disabled.

The returned amount need to include the watch-only
available balance too.

Solves #26687.

Author: furszy

src/qt/walletmodel.cpp

@@ -613,5 +613,17 @@ uint256 WalletModel::getLastBlockProcessed() const
 
 CAmount WalletModel::getAvailableBalance(const CCoinControl* control)
 {
-    return control && control->HasSelected() ? wallet().getAvailableBalance(*control) : getCachedBalance().balance;
+    // No selected coins, return the cached balance
+    if (!control || !control->HasSelected()) {
+        const interfaces::WalletBalances& balances = getCachedBalance();
+        CAmount available_balance = balances.balance;
+        // if wallet private keys are disabled, this is a watch-only wallet
+        // so, let's include the watch-only balance.
+        if (balances.have_watch_only && m_wallet->privateKeysDisabled()) {
+            available_balance += balances.watch_only_balance;
+        }
+        return available_balance;
+    }
+    // Fetch balance from the wallet, taking into account the selected coins
+    return wallet().getAvailableBalance(*control);
 }