Make 0,1 sats/vbyte available

Transaction Fee is currently only able to be 1 sat/vbyte. But it would be good to be able to set it to 0,1 sat/vbyte like sparrow offers as well.

No source file or test is named. Start by locating the transaction-fee setting and the validation or transaction-building entry point that handles sat/vbyte values, then compare the requested 0 and 0.1 options with Sparrow's behavior. Done means users can select 0 or 0.1 sat/vbyte and the resulting transaction uses that fee rate correctly.

Written by the indexing model from the issue text.