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/service/week_service.go

@@ -42,6 +42,21 @@ func NewWeekService(
 	}
 }
 
+// BalanceResult holds the overall overtime/undertime balance across all closed weeks.
+type BalanceResult struct {
+	TotalDeltaMs    int64 `json:"total_delta_ms"`
+	ClosedWeekCount int   `json:"closed_week_count"`
+}
+
+// Balance returns the sum of delta_ms across all closed weeks.
+func (s *WeekService) Balance(ctx context.Context) (BalanceResult, error) {
+	total, count, err := s.closedWeeks.SumDelta(ctx)
+	if err != nil {
+		return BalanceResult{}, err
+	}
+	return BalanceResult{TotalDeltaMs: total, ClosedWeekCount: count}, nil
+}
+
 // WeekDayKeysExported is exported for testing.
 var WeekDayKeysExported = weekDayKeys