frontend: show Energy/DSMR/meter times in local timezone, 24h

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).

frontend/src/pages/EnergyPage.tsx

@@ -48,6 +48,7 @@ import { DsmrPanel } from '../energy/DsmrPanel'
 import type { ModbusDevice, ModbusTestReadResponse, MetricInfo } from '../energy/hooks'
 import { ApiError } from '../api/client'
 import { formatMetricValue } from '../energy/format'
+import { formatLocalDateTime } from '../utils/datetime'
 
 // ---------------------------------------------------------------------------
 // Delete confirmation modal (with 409 "disable instead" hint)
@@ -311,7 +312,7 @@ function LatestReadingsCard({ device, autoRefresh }: LatestReadingsCardProps) {
           </Text>
           {latest.recorded_at && (
             <Text size="xs" c="dimmed">
-              {new Date(latest.recorded_at).toLocaleString()}
+              {formatLocalDateTime(latest.recorded_at)}
             </Text>
           )}
         </Group>