Fix overview screen balance number

[hsjoberg]

Currently, the balance number on the overview screen is non-nonsensical. The channel reserve is included in this number, which means it's not actually the amount you can send.

We should address this.

[hsjoberg]

My preferred solution is lowering or removing the channel reserve and then make this number what we show this number instead.

There are some issues about commitment fee, as this may change depending on the onchain fee market (if the channel initiator is the wallet).