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/TibberPrices.tsx

@@ -32,6 +32,7 @@ import {
   ResponsiveContainer,
 } from 'recharts'
 import { useEnergyPrices } from './hooks'
+import { formatLocalTime } from '../utils/datetime'
 
 // ---------------------------------------------------------------------------
 // Time range helpers
@@ -61,7 +62,7 @@ interface TibberChartProps {
 
 function TibberChart({ points, currency }: TibberChartProps) {
   const data = points.map((p) => ({
-    time: new Date(p.starts_at).toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' }),
+    time: formatLocalTime(p.starts_at),
     buy: p.buy,
     sell: p.sell,
   }))