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/energy/hooks.ts

@@ -385,13 +385,18 @@ export function useEnergyCostSummary(start?: string, end?: string) {
 // Query: DSMR latest reading
 // ---------------------------------------------------------------------------
 
-export function useDsmrLatest() {
+export function useDsmrLatest(options?: AutoRefreshOptions) {
+  const refetchInterval = options?.refetchIntervalMs != null
+    ? Math.max(2_000, options.refetchIntervalMs)
+    : undefined
+
   return useQuery({
     queryKey: ['dsmr-latest'],
     queryFn: async () => {
       const res = await apiClient.GET('/api/energy/dsmr/latest')
       return res.data
     },
+    refetchInterval,
   })
 }