Backend serializes SQLite-read (naive) UTC datetimes without a Z, so the browser
was parsing them as local time -> the UI effectively showed UTC values.
- New src/utils/datetime.ts: treats a tz-less timestamp as UTC, then formats in
the browser's local timezone with hour12:false (no AM/PM).
- Applied to Energy tabs (contracts/prices/costs/DSMR), per-device readings &
trends, and the meter readings table on the Records page.
- CostView Today/This month now use local day boundaries (queries stay UTC ISO).
- Untouched: location/poo (raw Zulu strings), the map, and query-window params.
- Tests are timezone-independent (verified under TZ=America/Los_Angeles).
