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
                     }

electrum/gui/qml/components/WalletMainView.qml

@@ -255,11 +255,11 @@ Item {
             height: parent.height
 
             onDoPay: {
-                if (invoice.invoiceType == Invoice.OnchainInvoice) {
+                if (invoice.invoiceType == Invoice.OnchainInvoice || (invoice.invoiceType == Invoice.LightningInvoice && invoice.amount.satsInt > Daemon.currentWallet.lightningCanSend ) ) {
                     var dialog = confirmPaymentDialog.createObject(mainView, {
-                            'address': invoice.address,
-                            'satoshis': invoice.amount,
-                            'message': invoice.message
+                            address: invoice.address,
+                            satoshis: invoice.amount,
+                            message: invoice.message
                     })
                     var canComplete = !Daemon.currentWallet.isWatchOnly && Daemon.currentWallet.canSignWithoutCosigner
                     dialog.txaccepted.connect(function() {

electrum/gui/qml/qeinvoice.py

@@ -27,8 +27,7 @@ class QEInvoice(QObject):
         Invalid = -1
         OnchainInvoice = 0
         LightningInvoice = 1
-        LightningAndOnchainInvoice = 2
-        LNURLPayRequest = 3
+        LNURLPayRequest = 2
 
     class Status:
         Unpaid = PR_UNPAID
@@ -307,6 +306,10 @@ class QEInvoiceParser(QEInvoice):
                         self.userinfo = _('Cannot pay less than the amount specified in the invoice')
                     else:
                         self.canPay = True
+                elif self.address and self.get_max_spendable_onchain() > self.amount.satsInt:
+                    # TODO: validate address?
+                    # TODO: subtract fee?
+                    self.canPay = True
                 else:
                     self.userinfo = _('Insufficient balance')
             else:
@@ -323,7 +326,7 @@ class QEInvoiceParser(QEInvoice):
                     # TODO: dust limit?
                     self.canPay = True
                 elif self.get_max_spendable_onchain() >= self.amount.satsInt:
-                    # TODO: dust limit?
+                    # TODO: subtract fee?
                     self.canPay = True
                 else:
                     self.userinfo = _('Insufficient balance')