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/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));