Epic: Heating improvements

Recently, we have added more and more heating devices: https://github.com/evcc-io/evcc/pulls?q=is%3Apr+is%3Aclosed+label%3Aheating. To simplify implementation we've provided two basic abstractions that allow quickly prototyping new models on top: heatpump(power-controller) and sgready(2-wire controlled).

Heating devices, especially heatpumps, are different than the usual EV charger in a couple of aspects:

• they are, typically continuously, controlled by power, not current
• operate independently of a given desired power envelope
• power consumption is not predictable
• behaviour does not easily map to enabled/disabled
• hence energy is also consumed when "disabled" and users want it to be logged
• charging status detection is not reliable (https://github.com/evcc-io/evcc/issues/19481#issuecomment-2705693911)
• planner logic independent from soc (https://github.com/evcc-io/evcc/issues/27396#issuecomment-3901804005)
• smart grid charging (https://github.com/evcc-io/evcc/issues/25546)
• improve state communication in ui (standby, sg boost, settings, ...) see https://github.com/evcc-io/evcc/issues/27010, https://github.com/evcc-io/evcc/issues/27733
• device if we want to allow user-controlled temp limits. see https://github.com/evcc-io/evcc/issues/27970
• lower priority for heating (usual use case: vehicles > home battery > heating) https://github.com/evcc-io/evcc/issues/29448

This epic serves to collect ideas how to improve the current functionality.

Loose summary of things we might potentially do next

• better status visualization for heatpumps (i.e. differentiate between "normal" and "boost" mode and understand enabled/disabled vs actually heating [or cooling/pumping]) https://github.com/evcc-io/evcc/pull/28025 https://github.com/evcc-io/evcc/pull/29667
• better tracking of heating device power consumption https://github.com/evcc-io/evcc/pull/29143
• track heating device boost start/stop times (aka mode)
• track heating device dimm state
• eventually technically split power and current controller
• to be decided: support heating energy amount/ x hours per day https://github.com/evcc-io/evcc/issues/29735