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/ContractManager.tsx

@@ -33,6 +33,7 @@ import {
 } from './hooks'
 import { ContractForm } from './ContractForm'
 import apiClient from '../api/client'
+import { formatLocalDate } from '../utils/datetime'
 
 // ---------------------------------------------------------------------------
 // Version history modal
@@ -96,11 +97,11 @@ function VersionHistoryModal({ contractId, contractName, onClose }: VersionHisto
                   <Accordion.Control>
                     <Group gap="sm">
                       <Text size="sm" fw={500}>
-                        From: {new Date(v.effective_from).toLocaleDateString()}
+                        From: {formatLocalDate(v.effective_from)}
                       </Text>
                       {v.effective_to ? (
                         <Text size="xs" c="dimmed">
-                          To: {new Date(v.effective_to).toLocaleDateString()}
+                          To: {formatLocalDate(v.effective_to)}
                         </Text>
                       ) : (
                         <Badge color="green" size="xs">
@@ -195,7 +196,7 @@ function ContractTable({
               <Table.Td>{contract.currency}</Table.Td>
               <Table.Td>
                 <Text size="xs" c="dimmed">
-                  {new Date(contract.created_at).toLocaleDateString()}
+                  {formatLocalDate(contract.created_at)}
                 </Text>
               </Table.Td>
               <Table.Td>