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_test.go

@@ -241,3 +241,48 @@ func TestWeekSnapshotUpdatesWhenDayReopened(t *testing.T) {
 		t.Errorf("updated delta_ms: want %dms, got %dms", wantDelta, cw2.DeltaMs)
 	}
 }
+
+func TestWeekServiceBalance(t *testing.T) {
+	ctx := context.Background()
+	_, daySvc, weekSvc, _ := newFullServices(t)
+
+	// Empty — no closed weeks yet.
+	bal, err := weekSvc.Balance(ctx)
+	if err != nil {
+		t.Fatalf("Balance (empty): %v", err)
+	}
+	if bal.TotalDeltaMs != 0 || bal.ClosedWeekCount != 0 {
+		t.Errorf("empty: want {0,0}, got {%d,%d}", bal.TotalDeltaMs, bal.ClosedWeekCount)
+	}
+
+	// Close two weeks as holiday (worked = expected → delta = 0 each).
+	for _, spec := range []struct {
+		weekKey string
+		monday  time.Time
+	}{
+		{"2024-W03", time.Date(2024, 1, 15, 0, 0, 0, 0, time.UTC)},
+		{"2024-W04", time.Date(2024, 1, 22, 0, 0, 0, 0, time.UTC)},
+	} {
+		for i := 0; i < 5; i++ {
+			dk := spec.monday.AddDate(0, 0, i).Format("2006-01-02")
+			if _, err := daySvc.MarkDay(ctx, dk, domain.DayKindHoliday); err != nil {
+				t.Fatalf("MarkDay %s: %v", dk, err)
+			}
+		}
+		if _, err := weekSvc.CloseWeek(ctx, spec.weekKey); err != nil {
+			t.Fatalf("CloseWeek %s: %v", spec.weekKey, err)
+		}
+	}
+
+	bal, err = weekSvc.Balance(ctx)
+	if err != nil {
+		t.Fatalf("Balance (populated): %v", err)
+	}
+	if bal.ClosedWeekCount != 2 {
+		t.Errorf("count: want 2, got %d", bal.ClosedWeekCount)
+	}
+	// Both weeks: worked == expected (holiday marks = full hours), delta = 0.
+	if bal.TotalDeltaMs != 0 {
+		t.Errorf("total_delta_ms: want 0, got %d", bal.TotalDeltaMs)
+	}
+}