qml: enable canPay in InvoiceDialog if wallet has insufficient funds to pay via lightning

and invoice has fallback address and amount can be paid on-chain.
In WalletMainView, follow on-chain payment path if available lighting balance is
insufficient for the invoice amount

electrum/gui/qml/components/InvoiceDialog.qml

@@ -78,7 +78,9 @@ ElDialog {
                         text: invoice.invoiceType == Invoice.OnchainInvoice
                                 ? qsTr('On chain')
                                 : invoice.invoiceType == Invoice.LightningInvoice
-                                    ? qsTr('Lightning')
+                                    ? invoice.address
+                                        ? qsTr('Lightning with on-chain fallback address')
+                                        : qsTr('Lightning')
                                     : ''
                         Layout.fillWidth: true
                     }