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/domain/domain.go

@@ -127,9 +127,13 @@ type Garden struct {
 	HeightCM  float64 `json:"heightCm"`
 	UnitPref  string  `json:"unitPref"`
 	Notes     string  `json:"notes"`
-	Version   int64   `json:"version"`
-	CreatedAt string  `json:"createdAt"`
-	UpdatedAt string  `json:"updatedAt"`
+	// GridSizeCM is the garden-scale grid spacing (cm) the editor draws its grid
+	// from; SnapToGrid, when true, snaps objects to that grid on place/move/resize.
+	GridSizeCM float64 `json:"gridSizeCm"`
+	SnapToGrid bool    `json:"snapToGrid"`
+	Version    int64   `json:"version"`
+	CreatedAt  string  `json:"createdAt"`
+	UpdatedAt  string  `json:"updatedAt"`
 
 	// MyRole is the requesting actor's effective role on this garden ("owner",
 	// "editor", or "viewer"). Computed by the service, never persisted.
@@ -174,7 +178,11 @@ type GardenObject struct {
 	Plantable   bool    `json:"plantable"`
 	Color       *string `json:"color,omitempty"`
 	Props       *string `json:"props,omitempty"` // kind-specific JSON
-	Notes       string  `json:"notes"`
+	// GridSizeCM is this object's local grid spacing (cm); when SnapToGrid is true
+	// and the object is a plantable bed, plants snap to it inside the bed.
+	GridSizeCM float64 `json:"gridSizeCm"`
+	SnapToGrid bool    `json:"snapToGrid"`
+	Notes      string  `json:"notes"`
 	Version     int64   `json:"version"`
 	CreatedAt   string  `json:"createdAt"`
 	UpdatedAt   string  `json:"updatedAt"`