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/store/objects.go

@@ -11,21 +11,23 @@ import (
 
 // objectColumns lists garden_objects columns in the order scanObject expects.
 const objectColumns = `id, garden_id, kind, name, shape, points, x_cm, y_cm, width_cm, height_cm,
-	rotation_deg, z_index, plantable, color, props, notes, version, created_at, updated_at`
+	rotation_deg, z_index, plantable, color, props, grid_size_cm, snap_to_grid, notes, version, created_at, updated_at`
 
 func scanObject(s scanner) (*domain.GardenObject, error) {
 	var (
 		o         domain.GardenObject
 		plantable int64
+		snap      int64
 	)
 	if err := s.Scan(
 		&o.ID, &o.GardenID, &o.Kind, &o.Name, &o.Shape, &o.Points,
 		&o.XCM, &o.YCM, &o.WidthCM, &o.HeightCM, &o.RotationDeg, &o.ZIndex,
-		&plantable, &o.Color, &o.Props, &o.Notes, &o.Version, &o.CreatedAt, &o.UpdatedAt,
+		&plantable, &o.Color, &o.Props, &o.GridSizeCM, &snap, &o.Notes, &o.Version, &o.CreatedAt, &o.UpdatedAt,
 	); err != nil {
 		return nil, err
 	}
 	o.Plantable = plantable != 0
+	o.SnapToGrid = snap != 0
 	return &o, nil
 }
 
@@ -35,11 +37,12 @@ func (d *DB) CreateObject(ctx context.Context, o *domain.GardenObject) (*domain.
 	created, err := scanObject(d.sql.QueryRowContext(ctx,
 		`INSERT INTO garden_objects
 		   (garden_id, kind, name, shape, points, x_cm, y_cm, width_cm, height_cm,
-		    rotation_deg, z_index, plantable, color, props, notes)
-		 VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
+		    rotation_deg, z_index, plantable, color, props, grid_size_cm, snap_to_grid, notes)
+		 VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
 		 RETURNING `+objectColumns,
 		o.GardenID, o.Kind, o.Name, o.Shape, o.Points, o.XCM, o.YCM, o.WidthCM, o.HeightCM,
-		o.RotationDeg, o.ZIndex, boolToInt(o.Plantable), o.Color, o.Props, o.Notes,
+		o.RotationDeg, o.ZIndex, boolToInt(o.Plantable), o.Color, o.Props,
+		o.GridSizeCM, boolToInt(o.SnapToGrid), o.Notes,
 	))
 	if err != nil {
 		return nil, fmt.Errorf("store: insert object: %w", err)
@@ -95,13 +98,14 @@ func (d *DB) UpdateObject(ctx context.Context, o *domain.GardenObject) (*domain.
 		`UPDATE garden_objects
 		    SET kind = ?, name = ?, shape = ?, points = ?, x_cm = ?, y_cm = ?,
 		        width_cm = ?, height_cm = ?, rotation_deg = ?, z_index = ?,
-		        plantable = ?, color = ?, props = ?, notes = ?,
+		        plantable = ?, color = ?, props = ?, grid_size_cm = ?, snap_to_grid = ?, notes = ?,
 		        version = version + 1,
 		        updated_at = strftime('%Y-%m-%dT%H:%M:%SZ', 'now')
 		  WHERE id = ? AND version = ?
 		  RETURNING `+objectColumns,
 		o.Kind, o.Name, o.Shape, o.Points, o.XCM, o.YCM, o.WidthCM, o.HeightCM,
-		o.RotationDeg, o.ZIndex, boolToInt(o.Plantable), o.Color, o.Props, o.Notes,
+		o.RotationDeg, o.ZIndex, boolToInt(o.Plantable), o.Color, o.Props,
+		o.GridSizeCM, boolToInt(o.SnapToGrid), o.Notes,
 		o.ID, o.Version,
 	))
 	if errors.Is(err, sql.ErrNoRows) {