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/gardens.go

@@ -10,29 +10,37 @@ import (
 )
 
 // gardenColumns lists the gardens columns in the fixed order scanGarden expects.
-const gardenColumns = `id, owner_id, name, width_cm, height_cm, unit_pref, notes, version, created_at, updated_at`
+const gardenColumns = `id, owner_id, name, width_cm, height_cm, unit_pref, notes, grid_size_cm, snap_to_grid, version, created_at, updated_at`
 
 func scanGarden(s scanner) (*domain.Garden, error) {
-	var g domain.Garden
+	var (
+		g    domain.Garden
+		snap int64
+	)
 	if err := s.Scan(
 		&g.ID, &g.OwnerID, &g.Name, &g.WidthCM, &g.HeightCM,
-		&g.UnitPref, &g.Notes, &g.Version, &g.CreatedAt, &g.UpdatedAt,
+		&g.UnitPref, &g.Notes, &g.GridSizeCM, &snap, &g.Version, &g.CreatedAt, &g.UpdatedAt,
 	); err != nil {
 		return nil, err
 	}
+	g.SnapToGrid = snap != 0
 	return &g, nil
 }
 
 // scanGardenWithRole reads a garden row plus a trailing my_role column into the
 // computed Garden.MyRole field (used by the actor-scoped list).
 func scanGardenWithRole(s scanner) (*domain.Garden, error) {
-	var g domain.Garden
+	var (
+		g    domain.Garden
+		snap int64
+	)
 	if err := s.Scan(
 		&g.ID, &g.OwnerID, &g.Name, &g.WidthCM, &g.HeightCM,
-		&g.UnitPref, &g.Notes, &g.Version, &g.CreatedAt, &g.UpdatedAt, &g.MyRole,
+		&g.UnitPref, &g.Notes, &g.GridSizeCM, &snap, &g.Version, &g.CreatedAt, &g.UpdatedAt, &g.MyRole,
 	); err != nil {
 		return nil, err
 	}
+	g.SnapToGrid = snap != 0
 	return &g, nil
 }
 
@@ -45,10 +53,10 @@ const maxGardensListed = 1000
 // set and validated by the service) and returns the stored row.
 func (d *DB) CreateGarden(ctx context.Context, g *domain.Garden) (*domain.Garden, error) {
 	created, err := scanGarden(d.sql.QueryRowContext(ctx,
-		`INSERT INTO gardens (owner_id, name, width_cm, height_cm, unit_pref, notes)
-		 VALUES (?, ?, ?, ?, ?, ?)
+		`INSERT INTO gardens (owner_id, name, width_cm, height_cm, unit_pref, notes, grid_size_cm, snap_to_grid)
+		 VALUES (?, ?, ?, ?, ?, ?, ?, ?)
 		 RETURNING `+gardenColumns,
-		g.OwnerID, g.Name, g.WidthCM, g.HeightCM, g.UnitPref, g.Notes,
+		g.OwnerID, g.Name, g.WidthCM, g.HeightCM, g.UnitPref, g.Notes, g.GridSizeCM, boolToInt(g.SnapToGrid),
 	))
 	if err != nil {
 		return nil, fmt.Errorf("store: insert garden: %w", err)
@@ -117,11 +125,12 @@ func (d *DB) UpdateGarden(ctx context.Context, g *domain.Garden) (*domain.Garden
 	updated, err := scanGarden(d.sql.QueryRowContext(ctx,
 		`UPDATE gardens
 		    SET name = ?, width_cm = ?, height_cm = ?, unit_pref = ?, notes = ?,
+		        grid_size_cm = ?, snap_to_grid = ?,
 		        version = version + 1,
 		        updated_at = strftime('%Y-%m-%dT%H:%M:%SZ', 'now')
 		  WHERE id = ? AND version = ?
 		  RETURNING `+gardenColumns,
-		g.Name, g.WidthCM, g.HeightCM, g.UnitPref, g.Notes, g.ID, g.Version,
+		g.Name, g.WidthCM, g.HeightCM, g.UnitPref, g.Notes, g.GridSizeCM, boolToInt(g.SnapToGrid), g.ID, g.Version,
 	))
 	if errors.Is(err, sql.ErrNoRows) {
 		// No row matched: distinguish "gone" from "stale version".