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/energy/DsmrPanel.tsx

@@ -26,6 +26,7 @@ import {
   Badge,
 } from '@mantine/core'
 import { useDsmrLatest } from './hooks'
+import { formatLocalDateTime } from '../utils/datetime'
 
 /** Default auto-refresh interval (ms) — DSMR ingest stores ~one row per 10 s. */
 const AUTO_REFRESH_INTERVAL_MS = 10_000
@@ -117,7 +118,7 @@ function DsmrContent({ isLoading, isError, data }: DsmrContentProps) {
         </Badge>
         {data.recorded_at && (
           <Text size="sm" c="dimmed" data-testid="dsmr-recorded-at">
-            recorded at {new Date(data.recorded_at).toLocaleString()}
+            recorded at {formatLocalDateTime(data.recorded_at)}
           </Text>
         )}
       </Group>