feat: Improve home overview calculation.

[just-seba]

As soon as a smart meter and a producer are present, the home power consumption can be calculated by combining production and grid power information. This is more accurate than adding up all consumers.

Examples:

Production and consumption from grid
Producer: 500W
Meter: 300W consumption from grid
Home: 800W

Production and export to grid
Producer: 500W
Meter: 300W export to grid
Home: 200W