feat: overall overtime balance on history page

Backend:
- ClosedWeekStore.SumDelta: single SQL aggregate returning total delta_ms and
  row count across all closed_weeks
- WeekService.Balance: thin passthrough returning BalanceResult{TotalDeltaMs, ClosedWeekCount}
- GET /api/weeks/balance handler; route registered alongside /weeks list/close/reopen
- Tests: store-level SumDelta (empty + populated), service-level Balance (empty + 2 weeks)

Frontend:
- weeks.balance() added to API client
- History page: balance card at top, fetched in parallel with existing data
- Loading state shows '—'; once loaded shows formatDelta value in green/red/gray
- Shows 'across N closed weeks' count alongside the value

internal/handler/week_handler.go

@@ -19,10 +19,21 @@ func NewWeekHandler(svc *service.WeekService) *WeekHandler {
 
 func (h *WeekHandler) Routes(r chi.Router) {
 	r.Get("/weeks", h.List)
+	r.Get("/weeks/balance", h.Balance)
 	r.Post("/weeks/{week_key}/close", h.Close)
 	r.Delete("/weeks/{week_key}/close", h.Reopen)
 }
 
+// Balance GET /api/weeks/balance
+func (h *WeekHandler) Balance(w http.ResponseWriter, r *http.Request) {
+	bal, err := h.svc.Balance(r.Context())
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, err.Error())
+		return
+	}
+	writeJSON(w, http.StatusOK, bal)
+}
+
 // List GET /api/weeks?from=YYYY-Www&to=YYYY-Www
 func (h *WeekHandler) List(w http.ResponseWriter, r *http.Request) {
 	from := r.URL.Query().Get("from")