frontend: add DSMR data panel to Energy page

- New 'DSMR' tab showing the latest parsed telegram (GET /api/energy/dsmr/latest)
  as a key/value table — the source-of-truth view so you can see ingest is
  landing data without reading the DB.
- Auto-refresh toggle (~10s); loading/error/empty states (empty explains the
  likely cause); null phase values shown as a dash.
- useDsmrLatest extended with optional auto-refresh. Tests added.

frontend/src/pages/EnergyPage.tsx

@@ -44,6 +44,7 @@ import { EnergyCharts } from '../energy/EnergyCharts'
 import { ContractManager } from '../energy/ContractManager'
 import { TibberPrices } from '../energy/TibberPrices'
 import { CostView } from '../energy/CostView'
+import { DsmrPanel } from '../energy/DsmrPanel'
 import type { ModbusDevice, ModbusTestReadResponse, MetricInfo } from '../energy/hooks'
 import { ApiError } from '../api/client'
 import { formatMetricValue } from '../energy/format'
@@ -628,6 +629,9 @@ export function EnergyPage() {
           <Tabs.Tab value="costs" data-testid="tab-costs">
             Costs
           </Tabs.Tab>
+          <Tabs.Tab value="dsmr" data-testid="tab-dsmr">
+            DSMR
+          </Tabs.Tab>
         </Tabs.List>
 
         <Tabs.Panel value="devices" data-testid="panel-devices">
@@ -645,6 +649,10 @@ export function EnergyPage() {
         <Tabs.Panel value="costs" data-testid="panel-costs">
           <CostView />
         </Tabs.Panel>
+
+        <Tabs.Panel value="dsmr" data-testid="panel-dsmr">
+          <DsmrPanel />
+        </Tabs.Panel>
       </Tabs>
     </Container>
   )