v0.2.1 — History tab polish

Fixes found by testing the statistics call against a live instance.

• Saving chart now warns about the model change. energy_saving_today has 31 days of statistics recorded under the pre-v0.2.0 model, which did not subtract battery charge cost. Charting those alongside corrected values shows a step down on 2026-09-05 that looks like a performance collapse but is purely a change in calculation. The chart says so.
• Empty charts explain themselves. Per-tariff grid import has no statistics until the meters complete their first full day; an empty Chart.js canvas rendered as a blank grid that read as broken.

Remember to bump the ?v= on panel_custom's module_url to 0.2.1 when you copy the package up — without it a CDN will keep serving the old file.