refactor: group uuid instead of index

internal/model/game/group_test.go

@@ -162,7 +162,7 @@ func TestShipGroupEqual(t *testing.T) {
 	mat := game.CargoMaterial
 	cap := game.CargoCapital
 	left := &game.ShipGroup{
-		Index:  1,
+		ID:     uuid.New(),
 		Number: 1,
 
 		OwnerID:   uuid.New(),
@@ -241,7 +241,7 @@ func TestShipGroupEqual(t *testing.T) {
 	// non-essential properties
 	right = *left
 
-	left.Index = 2
+	left.ID = uuid.New()
 	assert.True(t, left.Equal(right))
 
 	// dirty hack to equalize loads