qt: refactor message for change-to-lightning swap to backend submarine_swaps.py

diff --git a/electrum/gui/qt/confirm_tx_dialog.py b/electrum/gui/qt/confirm_tx_dialog.py
index edfa715..a53f36c 100644
--- a/electrum/gui/qt/confirm_tx_dialog.py
+++ b/electrum/gui/qt/confirm_tx_dialog.py
@@ -683,39 +683,11 @@ class TxEditor(WindowModalDialog, QtEventListener, Logger):
                 messages.append(long_warning)
         if self.no_dynfee_estimates:
             self.error = _('Fee estimates not available. Please set a fixed fee or feerate.')
-        if dummy_output := self.tx.get_dummy_output(DummyAddress.SWAP):
-            swap_msg = _('Will send change to lightning')
-            swap_fee_msg = "."
-            if self.swap_manager and self.swap_manager.is_initialized.is_set() and isinstance(dummy_output.value, int):
-                ln_amount_we_recv = self.swap_manager.get_recv_amount(send_amount=dummy_output.value, is_reverse=False)
-                if ln_amount_we_recv:
-                    swap_fees = dummy_output.value - ln_amount_we_recv
-                    swap_fee_msg = " [" + _("Swap fees:") + " " + self.main_window.format_amount_and_units(swap_fees) + "]."
-            messages.append(swap_msg + swap_fee_msg)
-        elif self.config.WALLET_SEND_CHANGE_TO_LIGHTNING \
-                and not (self.swap_transport and self.swap_transport.ongoing_connection_attempt) \
-                and self.tx.has_change():
-            swap_msg = _('Will not send change to Lightning')
-            swap_msg_reason = None
-            change_amount = sum(c.value for c in self.tx.get_change_outputs() if isinstance(c.value, int))
-            if not self.wallet.has_lightning():
-                swap_msg_reason = _('Lightning is not enabled.')
-            elif change_amount > int(self.wallet.lnworker.num_sats_can_receive()):
-                swap_msg_reason = _("Your channels cannot receive this amount.")
-            elif self.wallet.lnworker.swap_manager.is_initialized.is_set():
-                min_amount = self.wallet.lnworker.swap_manager.get_min_amount()
-                max_amount = self.wallet.lnworker.swap_manager.get_provider_max_reverse_amount()
-                if change_amount < min_amount:
-                    swap_msg_reason = _("Below the swap providers minimum value of {}.").format(
-                        self.main_window.format_amount_and_units(min_amount)
-                    )
-                else:
-                    swap_msg_reason = _('Change amount exceeds the swap providers maximum value of {}.').format(
-                        self.main_window.format_amount_and_units(max_amount)
-                    )
-            messages.append(swap_msg + (f": {swap_msg_reason}" if swap_msg_reason else '.'))
-        elif self.swap_transport and self.swap_transport.ongoing_connection_attempt:
-            messages.append(_("Fetching submarine swap providers..."))
+        if self.config.WALLET_SEND_CHANGE_TO_LIGHTNING:
+            if not self.swap_manager:
+                messages.append(_("Lightning is not enabled."))
+            elif swap_msg := self.swap_manager.get_message_for_swap_change(self.swap_transport, self.tx):
+                messages.append(swap_msg)
         # warn if spending unconf
         if any((txin.block_height is not None and txin.block_height<=0) for txin in self.tx.inputs()):
             messages.append(_('This transaction will spend unconfirmed coins.'))
diff --git a/electrum/submarine_swaps.py b/electrum/submarine_swaps.py
index 78aa2bb..9607f8a 100644
--- a/electrum/submarine_swaps.py
+++ b/electrum/submarine_swaps.py
@@ -372,6 +372,41 @@ class SwapManager(Logger):
         # we couldn't even connect to the relays, this transport is useless. maybe network issues.
         return False
 
+    def get_message_for_swap_change(self, swap_transport, tx):
+        """ UI support for send-change-to-lightning.
+        """
+        msg = ''
+        if swap_transport is not None and swap_transport.ongoing_connection_attempt:
+            msg = _("Fetching submarine swap providers...")
+        elif dummy_output := tx.get_dummy_output(DummyAddress.SWAP):
+            msg = _('Will send change to lightning')
+            if self.is_initialized.is_set() and isinstance(dummy_output.value, int):
+                ln_amount_we_recv = self.get_recv_amount(send_amount=dummy_output.value,
+                                                         is_reverse=False)
+                if ln_amount_we_recv:
+                    swap_fees = dummy_output.value - ln_amount_we_recv
+                    msg += " [" + _("Swap fees:") + " " + self.config.format_amount_and_units(swap_fees) + "]."
+        elif not tx.has_change():
+            msg = _('No change output, so no need for swap')
+        else:
+            change_amount = sum(c.value for c in tx.get_change_outputs() if isinstance(c.value, int))
+            if change_amount > int(self.wallet.lnworker.num_sats_can_receive()):
+                msg = _("Your channels cannot receive this amount.")
+            elif self.is_initialized.is_set():
+                min_amount = self.get_min_amount()
+                max_amount = self.get_provider_max_reverse_amount()
+                if change_amount < min_amount:
+                    msg = _("Below the swap providers minimum value of {}.").format(
+                        self.config.format_amount_and_units(min_amount)
+                    )
+                elif change_amount > max_amount:
+                    msg = _('Change amount exceeds the swap providers maximum value of {}.').format(
+                        self.config.format_amount_and_units(max_amount)
+                    )
+            else:
+                msg = _('Will not send change to Lightning')
+        return msg
+
     async def set_nostr_proof_of_work(self) -> None:
         current_pow = get_nostr_ann_pow_amount(
             self.lnworker.nostr_keypair.pubkey[1:],