fix: make meter_power cumulative across charging sessions

[jak]

Summary

• Fixes meter_power resetting to 0 between charging sessions. Homey's meter_power capability expects a cumulative value (like a utility meter) that never decreases.
• Tracks energy deltas between polls and accumulates them into a running total stored in the Homey device store, so it persists across app restarts.
• The API's per-session energy value is still used as the source, but the device now adds session deltas to the cumulative counter rather than reporting the raw session value.

Test plan

• Start a charge session and verify meter_power increases as energy is delivered
• Unplug and re-plug the car — verify meter_power does not reset to 0
• Restart the Homey app — verify meter_power retains its previous value from the device store

Closes #4

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