FU10: localize energy time math to server tz; accrue fixed/credit per elapsed local day

- Store UTC, compute day boundaries in server local timezone (new app/services/timezone.py).
- summarize: fixed fee / tax credit accrue only for elapsed whole local days,
  integrated across contract versions — no future-day inflation, no version-switch collapse.
- MQTT import_cost_total / export_revenue_total getters delegate to summarize (no inline
  apportionment), anchored at the active contract's earliest version effective_from so the
  total sensors stay monotonic and never jump backward.
- effective_from: naive datetimes interpreted as server-local wall-clock, then stored UTC.
- frontend ContractForm sends local-midnight naive datetime (was UTC midnight).
- get_costs_summary default window uses server-local "today".

frontend/src/energy/ContractForm.tsx

@@ -218,9 +218,13 @@ export function ContractForm({ contractId, defaultKind, onClose, onSaved }: Cont
     const values = buildNestedValues(sectionFields, fieldValues)
 
     try {
-      // Convert local date string to ISO datetime if provided
+      // Convert local date string to a naive local-midnight datetime string (no Z).
+      // The backend interprets naive datetimes as server local wall-clock time,
+      // so "2026-06-25T00:00:00" → CEST local midnight → stored as UTC 22:00Z.
+      // When effectiveFromStr is empty, fall back to the current instant (aware,
+      // sent as ISO with Z) so the backend stores it unchanged.
       const effectiveFromISO = effectiveFromStr
-        ? new Date(effectiveFromStr).toISOString()
+        ? `${effectiveFromStr}T00:00:00`
         : undefined
 
       if (isAddVersion) {