feat: configurable sensor display order

[KadenThomp36]

Closes #19.

Summary

Adds an `order` config option so users can reorder which sensors appear first on the card.

Implementation

Uses flexbox `style.order` on each graph container rather than rebuilding the DOM — `.graphs` is already `display: flex` so this is a single-property set per metric and visually reflows them.

order:
  - temperature
  - humidity
  - co2
  - pm10
  - pm25

UX choices

• Unmentioned metrics are preserved, appended in default order. Users never lose a sensor by forgetting it in `order` — important for an opt-in customization where forgetting one entry shouldn't silently hide a sensor.
• Invalid metric names are dropped (no error), valid ones still applied.
• Non-array input falls back to defaults (no error).

Test plan

• `node test.js` — 221 passed, 0 failed (12 new: default order, full user override, partial override with unmentioned metrics, invalid metric names dropped, non-array input fallback, empty array fallback)
• Manual smoke test in HA: configure the example from the issue (`T, H, CO2, PM10, PM2.5`) → verify the card visually reorders

Notes for maintainer

• No `CARD_VERSION` bump — rolls into rc2
• Independent of all other open PRs — no expected conflicts