v1.9.1 — Expose real should-be-on decision for dashboards

New attributes on each device's power-tracking sensor: sollte_an_sein (the cascade's current on/off target for that device) and korrekt_geschaltet (whether the real device state matches it).

Dashboards previously had no choice but to reimplement an approximation of the switching logic to show "is this device correctly on/off" — which drifts out of sync as the real logic grows more sophisticated (per-device forward-looking battery check, dependencies, time windows, minimum runtime). Confirmed this actually happened on a real install: a hand-written dashboard formula said a device "should be on" for 3 hours overnight while the real coordinator had correctly kept it off, because the dashboard's simplified formula didn't know about the newer per-device projection logic.

These attributes are computed directly from the same should_on/should_off values the coordinator actually acts on, so a dashboard status card can read the real verdict instead of guessing.