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/objects_test.go

@@ -48,6 +48,41 @@ func TestCreateObjectDefaults(t *testing.T) {
 	}
 }
 
+func TestObjectGridSettings(t *testing.T) {
+	s := newTestService(t, openConfig())
+	owner := seedUser(t, s, "[email protected]")
+	g := seedGarden(t, s, owner)
+	ctx := context.Background()
+
+	// An omitted grid size defaults; snap defaults off.
+	o, err := s.CreateObject(ctx, owner, g.ID, ObjectInput{
+		Kind: domain.KindBed, XCM: 500, YCM: 500, WidthCM: 200, HeightCM: 100,
+	})
+	if err != nil {
+		t.Fatalf("CreateObject: %v", err)
+	}
+	if o.GridSizeCM != defaultObjectGridCM {
+		t.Errorf("grid = %v, want default %d", o.GridSizeCM, defaultObjectGridCM)
+	}
+	if o.SnapToGrid {
+		t.Error("snap should default off")
+	}
+
+	// Patching grid + snap round-trips and leaves other fields untouched.
+	grid := 15.0
+	snap := true
+	up, err := s.UpdateObject(ctx, owner, o.ID, ObjectPatch{GridSizeCM: &grid, SnapToGrid: &snap}, o.Version)
+	if err != nil {
+		t.Fatalf("UpdateObject: %v", err)
+	}
+	if up.GridSizeCM != 15 || !up.SnapToGrid {
+		t.Errorf("after patch grid=%v snap=%v, want 15/true", up.GridSizeCM, up.SnapToGrid)
+	}
+	if up.WidthCM != 200 || up.HeightCM != 100 {
+		t.Errorf("a grid patch changed dimensions: %vx%v", up.WidthCM, up.HeightCM)
+	}
+}
+
 func TestCreateObjectPlantableByKind(t *testing.T) {
 	s := newTestService(t, openConfig())
 	owner := seedUser(t, s, "[email protected]")
@@ -105,6 +140,7 @@ func TestCreateObjectValidation(t *testing.T) {
 		{Kind: domain.KindBed, XCM: 500, YCM: 500, WidthCM: 100, HeightCM: 100, Color: strPtr("nope")},                         // bad color
 		{Kind: domain.KindBed, XCM: 500, YCM: 500, WidthCM: 100, HeightCM: 100, Props: strPtr("{not json")},                    // bad props
 		{Kind: domain.KindBed, XCM: 500, YCM: 500, WidthCM: 100, HeightCM: 100, Name: strings.Repeat("x", maxObjectNameLen+1)}, // name too long
+		{Kind: domain.KindBed, XCM: 500, YCM: 500, WidthCM: 100, HeightCM: 100, GridSizeCM: maxGardenCM + 1},                   // grid over the cap
 	}
 	for i, in := range bad {
 		if _, err := s.CreateObject(context.Background(), owner, g.ID, in); !errors.Is(err, domain.ErrInvalidInput) {