If SwapManager.percentage was a 0.2 float, rounding differences would
cause an exception in the fee calculation inverse sanity check when entering 20
000 sats into the SwapDialog. By making self.percentage a decimal we can
prevent this kind of issue.
```
File "/home/user/code/vibecoding_vm/electrum/electrum/gui/qt/swap_dialog.py", line 294, in on_send_edited
recv_amount = self.swap_manager.get_recv_amount(send_amount, is_reverse=self.is_reverse)
File "/home/user/code/vibecoding_vm/electrum/electrum/submarine_swaps.py", line 1320, in get_recv_amount
if abs(send_amount - inverted_send_amount) > 1:
~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
TypeError: unsupported operand type(s) for -: 'int' and 'NoneType'
```
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