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/domain/domain.go

@@ -120,16 +120,20 @@ type Session struct {
 
 // Garden is a planning surface owned by one user, at real-world scale (cm).
 type Garden struct {
-	ID        int64   `json:"id"`
-	OwnerID   int64   `json:"ownerId"`
-	Name      string  `json:"name"`
-	WidthCM   float64 `json:"widthCm"`
-	HeightCM  float64 `json:"heightCm"`
-	UnitPref  string  `json:"unitPref"`
-	Notes     string  `json:"notes"`
-	Version   int64   `json:"version"`
-	CreatedAt string  `json:"createdAt"`
-	UpdatedAt string  `json:"updatedAt"`
+	ID       int64   `json:"id"`
+	OwnerID  int64   `json:"ownerId"`
+	Name     string  `json:"name"`
+	WidthCM  float64 `json:"widthCm"`
+	HeightCM float64 `json:"heightCm"`
+	UnitPref string  `json:"unitPref"`
+	Notes    string  `json:"notes"`
+	// 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.
@@ -174,10 +178,14 @@ type GardenObject struct {
 	Plantable   bool    `json:"plantable"`
 	Color       *string `json:"color,omitempty"`
 	Props       *string `json:"props,omitempty"` // kind-specific JSON
-	Notes       string  `json:"notes"`
-	Version     int64   `json:"version"`
-	CreatedAt   string  `json:"createdAt"`
-	UpdatedAt   string  `json:"updatedAt"`
+	// 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"`
 }
 
 // Plant is a catalog entry. OwnerID nil means a read-only seeded built-in.