Is your feature request related to a problem? Please describe.
When using a Generic / offline vehicle (no vehicle API), evcc has no way to account for vehicle pre-heating. Users typically keep such vehicles in PV or MinPV mode, which limits charging to minimum current unless PV surplus is available. During a pre-heating window, however, the vehicle requires higher charging power to supply cabin heating. Since evcc cannot detect pre-heating for offline vehicles, the planner does not temporarily increase charging power, leading to insufficient power during pre-heating or pre-heating not working at all.

Describe the solution you'd like
Add an optional, configurable pre-heating time to the charging planner for Generic / offline vehicles. If activated (checkbox in planner), during the configured time (as supplied via the configuration) before the planned leave (as set via the planner), evcc temporarily allows higher charging power (e.g. behave like Fast mode) so that pre-heating is reliably supplied with energy.

This feature could be enabled only if a certain additional vehicle config field is set (e.g., to set the pre-heating time) . If this feature is not useful to vehicles with APIs, it could just be available for offline` (generic) vehicles.

Describe alternatives you've considered

• Manually switching to Fast mode before each pre-heating event (not automatable, error-prone)
• Over-provisioning minimum current permanently (reduces PV self-consumption efficiency)

None of these integrate cleanly with evcc’s planner or charging philosophy.

Additional context
This feature would mainly benefit vehicles without APIs, where evcc cannot automatically detect pre-heating states. If I understand it correctly, API-enabled vehicles already handle this more intelligently (can’t test, don’t have such a car). A simple, declarative configuration would significantly improve usability for offline vehicles while keeping existing behavior unchanged by default.