Boost Charge slider to include negative numbers

[nathanmarlor]

Discussed in https://github.com/nathanmarlor/foxess_em/discussions/298

^{Originally posted by JohnHL12 June 7, 2023} Just a suggestion but the recent addition of the boost charge slide was a great idea, however how difficult would it be to let this go into negative numbers. For example in my home the weekend tends to be far higher usage, however come Monday everyone is back at work. This higher usage on the weekend bumps up the house load meaning that the batteries tend to be over charged early on in the week while things rebalance. The ability to reduce the required charge in this kind of situation would be rather useful or if people know they are going to be out all day the next day etc so will use far less power than normal.