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/EnergyCharts.tsx

@@ -43,6 +43,7 @@ import {
 import { useReadings, useMetrics } from './hooks'
 import type { MetricInfo, AutoRefreshOptions } from './hooks'
 import { formatMetricValue } from './format'
+import { formatLocalTime, formatLocalDateTime } from '../utils/datetime'
 
 // ---------------------------------------------------------------------------
 // Constants
@@ -107,12 +108,7 @@ function lineColor(index: number): string {
 // ---------------------------------------------------------------------------
 
 function formatTimeTick(isoString: string): string {
-  try {
-    const d = new Date(isoString)
-    return d.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' })
-  } catch {
-    return isoString
-  }
+  return formatLocalTime(isoString)
 }
 
 // ---------------------------------------------------------------------------
@@ -300,11 +296,7 @@ export function EnergyCharts({ uuid, deviceName, autoRefresh }: EnergyChartsProp
                           ] as [string, string]
                         }}
                         labelFormatter={(label) => {
-                          try {
-                            return new Date(String(label)).toLocaleString()
-                          } catch {
-                            return String(label)
-                          }
+                          return formatLocalDateTime(String(label))
                         }}
                       />
                       <Legend />