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/api/objects.go

@@ -27,6 +27,8 @@ type objectCreateRequest struct {
 	Plantable   *bool           `json:"plantable"`
 	Color       *string         `json:"color"`
 	Props       json.RawMessage `json:"props"`
+	GridSizeCM  float64         `json:"gridSizeCm"`
+	SnapToGrid  bool            `json:"snapToGrid"`
 	Notes       string          `json:"notes"`
 }
 
@@ -35,7 +37,8 @@ func (r objectCreateRequest) toInput() service.ObjectInput {
 		Kind: r.Kind, Name: r.Name, Shape: r.Shape,
 		XCM: r.XCM, YCM: r.YCM, WidthCM: r.WidthCM, HeightCM: r.HeightCM,
 		RotationDeg: r.RotationDeg, ZIndex: r.ZIndex, Plantable: r.Plantable,
-		Color: r.Color, Props: propsFromRaw(r.Props), Notes: r.Notes,
+		Color: r.Color, Props: propsFromRaw(r.Props),
+		GridSizeCM: r.GridSizeCM, SnapToGrid: r.SnapToGrid, Notes: r.Notes,
 	}
 }
 
@@ -54,6 +57,8 @@ type objectUpdateRequest struct {
 	Plantable   *bool           `json:"plantable"`
 	Color       json.RawMessage `json:"color"`
 	Props       json.RawMessage `json:"props"`
+	GridSizeCM  *float64        `json:"gridSizeCm"`
+	SnapToGrid  *bool           `json:"snapToGrid"`
 	Notes       *string         `json:"notes"`
 	Version     int64           `json:"version" binding:"required,min=1"`
 }
@@ -67,7 +72,8 @@ func (r objectUpdateRequest) toPatch() (service.ObjectPatch, error) {
 	return service.ObjectPatch{
 		Name: r.Name, XCM: r.XCM, YCM: r.YCM, WidthCM: r.WidthCM, HeightCM: r.HeightCM,
 		RotationDeg: r.RotationDeg, ZIndex: r.ZIndex, Plantable: r.Plantable,
-		SetColor: setColor, Color: color, SetProps: setProps, Props: props, Notes: r.Notes,
+		SetColor: setColor, Color: color, SetProps: setProps, Props: props,
+		GridSizeCM: r.GridSizeCM, SnapToGrid: r.SnapToGrid, Notes: r.Notes,
 	}, nil
 }