Cumulative reactive energy (kVARh) should use lowercase kvarh

[cpms67]

The P1 Reader component currently reports cumulative reactive energy (import/export) using the unit of measurement "kVARh" (uppercase VAR).

However, Home Assistant’s device class validation for reactive_energy only accepts lowercase "kvarh".

This causes warnings in Home Assistant logs, for example:

Entity sensor.esp8266_1_cumulative_reactive_import
is using native unit of measurement 'kVARh' which is not a valid unit
for the device class ('reactive_energy'); expected one of ['varh', 'kvarh', 'mvarh'].

Steps to reproduce:

Configure ESPHome with P1 Reader

Upload firmware

Observe Home Assistant logs → warning about invalid unit "kVARh"

Expected behavior:
Reactive energy should use the lowercase unit "kvarh".

Workaround tested:
In a local override of p1reader.cpp, I changed:

unit_of_measurement = "kVARh";

to:

unit_of_measurement = "kvarh";

After recompiling, Home Assistant accepted the unit and warnings disappeared.
ESPHome logs now show "kvarh" as expected.

Environment:

Home Assistant Core 2025.10.0

ESPHome 2025.9.3

P1 Reader version: latest

Hardware: ESP8266-1

Suggested fix:
Update p1reader.cpp to use "kvarh" instead of "kVARh" for cumulative reactive energy sensors.