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/store/closed_week_store.go

@@ -65,6 +65,14 @@ func (s *ClosedWeekStore) ListByRange(ctx context.Context, fromWeekKey, toWeekKe
 	return result, rows.Err()
 }
 
+// SumDelta returns the sum of delta_ms and the count of rows across all closed_weeks.
+func (s *ClosedWeekStore) SumDelta(ctx context.Context) (totalDeltaMs int64, count int, err error) {
+	err = s.db.QueryRowContext(ctx,
+		`SELECT COALESCE(SUM(delta_ms), 0), COUNT(*) FROM closed_weeks`,
+	).Scan(&totalDeltaMs, &count)
+	return
+}
+
 // SumWorkedMsForWeek sums worked_ms across closed_days for the given day keys.
 func SumWorkedMsForWeek(ctx context.Context, db *sql.DB, dayKeys []string) (int64, error) {
 	if len(dayKeys) == 0 {

internal/store/closed_week_store_test.go

@@ -0,0 +1,53 @@
+package store_test
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/wotra/wotra/internal/domain"
+	"github.com/wotra/wotra/internal/store"
+)
+
+func TestClosedWeekStoreSumDelta(t *testing.T) {
+	db, err := store.Open(":memory:")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	s := store.NewClosedWeekStore(db)
+	ctx := context.Background()
+	now := time.Now().UnixMilli()
+
+	// Empty table → 0, 0
+	total, count, err := s.SumDelta(ctx)
+	if err != nil {
+		t.Fatalf("SumDelta on empty table: %v", err)
+	}
+	if total != 0 || count != 0 {
+		t.Fatalf("empty: want (0,0), got (%d,%d)", total, count)
+	}
+
+	// Insert two weeks with known deltas.
+	for _, w := range []*domain.ClosedWeek{
+		{WeekKey: "2024-W01", ExpectedMs: 144_000_000, WorkedMs: 147_600_000, DeltaMs: 3_600_000, ClosedAt: now, UpdatedAt: now},
+		{WeekKey: "2024-W02", ExpectedMs: 144_000_000, WorkedMs: 142_200_000, DeltaMs: -1_800_000, ClosedAt: now, UpdatedAt: now},
+	} {
+		if err := s.Upsert(ctx, w); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	total, count, err = s.SumDelta(ctx)
+	if err != nil {
+		t.Fatalf("SumDelta: %v", err)
+	}
+	if count != 2 {
+		t.Errorf("count: want 2, got %d", count)
+	}
+	want := int64(3_600_000 - 1_800_000) // 1_800_000
+	if total != want {
+		t.Errorf("total: want %d, got %d", want, total)
+	}
+}