Configurable grid + snap-to-grid in the layout system (#45)

Closes #44. Two independent grids (garden + per-bed), each with a size and a snap toggle; snapping defaults off. See PR #45 for details.

Co-authored-by: Steve Dudenhoeffer <[email protected]>

internal/store/migrations/0005_grid_settings.sql

@@ -0,0 +1,17 @@
+-- Grid + snap settings for the layout system.
+--
+-- Two independent grids:
+--   * gardens.grid_size_cm / snap_to_grid — the garden-scale grid the visual grid
+--     is drawn from and (when snap is on) objects snap to when placed/moved/resized.
+--     Defaults to 100 cm (1 m), matching the editor's previous fixed grid.
+--   * garden_objects.grid_size_cm / snap_to_grid — a per-object grid used inside a
+--     plantable bed: when snap is on, plants snap to it. Defaults to 30 cm (~1 ft),
+--     a practical bed cell.
+--
+-- snap defaults to off on both, so existing gardens keep free placement until the
+-- owner (garden grid) or an editor (bed grid) opts in.
+ALTER TABLE gardens ADD COLUMN grid_size_cm REAL    NOT NULL DEFAULT 100;
+ALTER TABLE gardens ADD COLUMN snap_to_grid INTEGER NOT NULL DEFAULT 0 CHECK (snap_to_grid IN (0, 1));
+
+ALTER TABLE garden_objects ADD COLUMN grid_size_cm REAL    NOT NULL DEFAULT 30;
+ALTER TABLE garden_objects ADD COLUMN snap_to_grid INTEGER NOT NULL DEFAULT 0 CHECK (snap_to_grid IN (0, 1));