Robust entity discovery via registry and visible demo-mode notice

[Optic00]

Summary

The card discovers its data by guessing exact entity IDs (sensor.bms_1_cells_average_voltage, sensor.bmu_power, ...) and silently renders demo data when nothing matches. This PR makes discovery robust and the demo fallback visible:

• Discovery now goes through the entity registry (which the card already fetches in _ensureRegistries() for product names): entries are filtered by platform === 'byd_battery_box', then matched with relaxed patterns that tolerate any entity_id prefix and the numeric duplicate suffix HA appends (..._2). Without registry access (e.g. card preview), it falls back to the previous state-key scan, with the same relaxed patterns.
• Sibling sensors (min/max history, balancing, temperature, SoC, SoH, charge/discharge energy, efficiency) are resolved with the same matcher instead of literal template strings, and the matcher prefers entities with a usable state so an orphaned unavailable duplicate never shadows the live entity.
• The render change-detector accepts prefixed entity IDs too and now also fires when previously present sensors disappear, so the demo notice can appear without a reload.
• When discovery still finds nothing, the demo render is kept (useful for the card picker preview), but the card now shows a visible notice: "No byd_battery_box entities found – showing demo data".

Fixes #4.

Why exact-ID matching breaks

• HA appends _2 to entity IDs when the old registry entries still hold the original names. This happens whenever the byd_battery_box integration is removed and re-added, which is exactly the workaround users perform for First-time config entry setup fails with setup_error (no auto-retry) — should raise ConfigEntryNotReady on transient Modbus connect byd_battery_box#14 (first-time setup fails into setup_error; fix in Raise ConfigEntryNotReady on transient connect failure (fixes #14) byd_battery_box#15). After that, /^sensor\.bms_(\d+)_cells_average_voltage$/ no longer matches anything and the card quietly shows fake values (SoC 75 %, "BMS demo", 0 W).
• Renamed or prefixed entity IDs fail the same way.
• When the integration is not running at all, the card also pretends everything is fine instead of hinting at the problem.

Testing

• Static verification: node --check on source and rebuilt bundle; npm run build regenerated the committed bundle (CI's up-to-date check passes).
• Not yet run against a live HA instance. The manual matrix to verify is: default IDs -> real data exactly as before (exact matches still win); IDs with _2 suffix -> real data (previously demo); prefixed IDs -> real data (previously demo); integration absent or not running -> demo data plus visible notice.