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.go

@@ -10,9 +10,10 @@ import (
 )
 
 const (
-	maxObjectNameLen  = 200
-	maxObjectNotesLen = 10_000
-	maxObjectPropsLen = 20_000
+	maxObjectNameLen    = 200
+	maxObjectNotesLen   = 10_000
+	maxObjectPropsLen   = 20_000
+	defaultObjectGridCM = 30 // ~1 ft, a practical bed cell for plant snapping
 )
 
 // objectKinds maps each valid garden_objects.kind to its traits. plantable is
@@ -43,6 +44,8 @@ type ObjectInput struct {
 	Plantable   *bool
 	Color       *string
 	Props       *string
+	GridSizeCM  float64
+	SnapToGrid  bool
 	Notes       string
 }
 
@@ -65,6 +68,8 @@ type ObjectPatch struct {
 	Color       *string
 	SetProps    bool
 	Props       *string
+	GridSizeCM  *float64
+	SnapToGrid  *bool
 	Notes       *string
 }
 
@@ -110,6 +115,8 @@ func (s *Service) CreateObject(ctx context.Context, actorID, gardenID int64, in
 		Plantable:   plantable,
 		Color:       in.Color,
 		Props:       in.Props,
+		GridSizeCM:  in.GridSizeCM,
+		SnapToGrid:  in.SnapToGrid,
 		Notes:       strings.TrimSpace(in.Notes),
 	}
 	if err := finalizeObject(o, g); err != nil {
@@ -229,6 +236,12 @@ func applyObjectPatch(o *domain.GardenObject, p ObjectPatch) {
 	if p.SetProps {
 		o.Props = p.Props
 	}
+	if p.GridSizeCM != nil {
+		o.GridSizeCM = *p.GridSizeCM
+	}
+	if p.SnapToGrid != nil {
+		o.SnapToGrid = *p.SnapToGrid
+	}
 	if p.Notes != nil {
 		o.Notes = strings.TrimSpace(*p.Notes)
 	}
@@ -251,6 +264,15 @@ func finalizeObject(o *domain.GardenObject, g *domain.Garden) error {
 	if !validDimensionCM(o.WidthCM) || !validDimensionCM(o.HeightCM) {
 		return domain.ErrInvalidInput
 	}
+	// Grid spacing shares the dimension range [1 cm, 100 m]; 0 means "unset" and
+	// is defaulted so a create (or an older client omitting it) still lands a
+	// usable bed grid.
+	if o.GridSizeCM == 0 {
+		o.GridSizeCM = defaultObjectGridCM
+	}
+	if !validDimensionCM(o.GridSizeCM) {
+		return domain.ErrInvalidInput
+	}
 	if !isFinite(o.XCM) || !isFinite(o.YCM) || !isFinite(o.RotationDeg) {
 		return domain.ErrInvalidInput
 	}