Add configurable grid + snap to the layout system

Two independent grids, each with a size and a snap toggle:

- Garden grid (owner-set, in the garden settings form): drives the
  editor's visual grid and, when snapping is on, snaps objects on
  place/move and snaps their dimensions to whole grid steps on resize.
- Bed grid (per plantable object, in the bed Inspector): when snapping
  is on, plants snap to the bed's grid on place/move, and the grid is
  drawn inside the focused bed. Plant radius stays free.

Snapping defaults off on both, so existing gardens keep free placement.
Grid spacing shares the [1cm, 100m] range and defaults to 1m (garden) /
30cm (bed); 0 is treated as unset and re-defaulted.

Backend: migration 0005 adds grid_size_cm/snap_to_grid to gardens and
garden_objects, threaded through domain/store/service/api with service +
api round-trip tests. Frontend: new geometry snap helpers (unit-tested),
schema/type plumbing, canvas + overlay snapping, and the two forms.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

internal/service/gardens_test.go

@@ -57,6 +57,7 @@ func TestCreateGardenValidation(t *testing.T) {
 		{Name: "X", WidthCM: math.NaN()},                             // NaN slips past naive < / >
 		{Name: "X", HeightCM: math.Inf(1)},                           // +Inf
 		{Name: "X", WidthCM: math.SmallestNonzeroFloat64},            // subnormal, > 0 but < 1 cm
+		{Name: "X", GridSizeCM: maxGardenCM + 1},                     // grid over the cap
 	}
 	for i, in := range cases {
 		if _, err := s.CreateGarden(context.Background(), owner, in); !errors.Is(err, domain.ErrInvalidInput) {
@@ -70,6 +71,51 @@ func TestCreateGardenValidation(t *testing.T) {
 	}
 }
 
+func TestGardenGridSettings(t *testing.T) {
+	s := newTestService(t, openConfig())
+	owner := seedUser(t, s, "[email protected]")
+	ctx := context.Background()
+
+	// An omitted grid size lands the default; snap defaults off.
+	g, err := s.CreateGarden(ctx, owner, GardenInput{Name: "G"})
+	if err != nil {
+		t.Fatalf("CreateGarden: %v", err)
+	}
+	if g.GridSizeCM != defaultGardenGridCM {
+		t.Errorf("grid = %v, want default %d", g.GridSizeCM, defaultGardenGridCM)
+	}
+	if g.SnapToGrid {
+		t.Error("snap should default off")
+	}
+
+	// An update sets an explicit grid and turns snap on; both persist.
+	up, err := s.UpdateGarden(ctx, owner, g.ID, GardenInput{
+		Name: "G", WidthCM: g.WidthCM, HeightCM: g.HeightCM, UnitPref: g.UnitPref,
+		GridSizeCM: 25, SnapToGrid: true,
+	}, g.Version)
+	if err != nil {
+		t.Fatalf("UpdateGarden: %v", err)
+	}
+	got, err := s.GetGarden(ctx, owner, g.ID)
+	if err != nil {
+		t.Fatalf("GetGarden: %v", err)
+	}
+	if got.GridSizeCM != 25 || !got.SnapToGrid {
+		t.Errorf("persisted grid=%v snap=%v, want 25/true", got.GridSizeCM, got.SnapToGrid)
+	}
+
+	// An update omitting the grid size (0) is re-defaulted, not stored as 0.
+	up2, err := s.UpdateGarden(ctx, owner, g.ID, GardenInput{
+		Name: "G", WidthCM: g.WidthCM, HeightCM: g.HeightCM, UnitPref: g.UnitPref,
+	}, up.Version)
+	if err != nil {
+		t.Fatalf("UpdateGarden(reset): %v", err)
+	}
+	if up2.GridSizeCM != defaultGardenGridCM || up2.SnapToGrid {
+		t.Errorf("after omit grid=%v snap=%v, want default/false", up2.GridSizeCM, up2.SnapToGrid)
+	}
+}
+
 func TestListGardensOwnedOnly(t *testing.T) {
 	s := newTestService(t, openConfig())
 	alice := seedUser(t, s, "[email protected]")