swaps: (fix) for forward swaps, correctly consider num_sats_can_receive

Previously the min() was passed lightning amounts and on-chain amounts mixed;
which is conceptually a type error. It is now only passed on-chain amounts.
Due to the bug, we did not allow a swap to fully exhaust out "LN receive" capacity.
Now the max amt can be slighly larger.

electrum/gui/kivy/uix/dialogs/lightning_channels.py

@@ -726,7 +726,9 @@ class SwapDialog(Factory.Popup):
             max_onchain_spend = 0
         reverse = int(min(self.lnworker.num_sats_can_send(),
                           self.swap_manager.get_max_amount()))
-        forward = int(min(self.swap_manager.num_sats_can_receive(),
+        max_recv_amt_ln = int(self.swap_manager.num_sats_can_receive())
+        max_recv_amt_oc = self.swap_manager.get_send_amount(max_recv_amt_ln, is_reverse=False) or float('inf')
+        forward = int(min(max_recv_amt_oc,
                           # maximally supported swap amount by provider
                           self.swap_manager.get_max_amount(),
                           max_onchain_spend))

electrum/gui/qt/swap_dialog.py

@@ -126,8 +126,9 @@ class SwapDialog(WindowModalDialog):
         if self.tx:
             amount = self.tx.output_value_for_address(ln_dummy_address())
             max_swap_amt = self.swap_manager.get_max_amount()
-            max_recv_amt = int(self.swap_manager.num_sats_can_receive())
-            max_amt = min(max_swap_amt, max_recv_amt)
+            max_recv_amt_ln = int(self.swap_manager.num_sats_can_receive())
+            max_recv_amt_oc = self.swap_manager.get_send_amount(max_recv_amt_ln, is_reverse=False) or float('inf')
+            max_amt = int(min(max_swap_amt, max_recv_amt_oc))
             if amount > max_amt:
                 amount = max_amt
                 self._update_tx(amount)