Refactored to use ESPHome's existing DEVICE_CLASS_* constants instead
of creating duplicate custom constants. This makes the component more
consistent with the rest of ESPHome and reduces code duplication.
Changes:
- Import DEVICE_CLASS_* constants from esphome.const
- Map device classes to BTHome object IDs in dictionaries
- Remove custom CONF_* duplicates (battery, temperature, motion, etc.)
- Use standard device class strings ("temperature", "humidity", etc.)