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/api/gardens.go

@@ -14,15 +14,20 @@ import (
 // Dimensions are centimeters (the API is metric-only; imperial is a display
 // concern). On create, omitted dimensions default server-side.
 type gardenFields struct {
-	Name     string  `json:"name" binding:"required"`
-	WidthCM  float64 `json:"widthCm"`
-	HeightCM float64 `json:"heightCm"`
-	UnitPref string  `json:"unitPref"`
-	Notes    string  `json:"notes"`
+	Name       string  `json:"name" binding:"required"`
+	WidthCM    float64 `json:"widthCm"`
+	HeightCM   float64 `json:"heightCm"`
+	UnitPref   string  `json:"unitPref"`
+	Notes      string  `json:"notes"`
+	GridSizeCM float64 `json:"gridSizeCm"`
+	SnapToGrid bool    `json:"snapToGrid"`
 }
 
 func (f gardenFields) toInput() service.GardenInput {
-	return service.GardenInput{Name: f.Name, WidthCM: f.WidthCM, HeightCM: f.HeightCM, UnitPref: f.UnitPref, Notes: f.Notes}
+	return service.GardenInput{
+		Name: f.Name, WidthCM: f.WidthCM, HeightCM: f.HeightCM, UnitPref: f.UnitPref, Notes: f.Notes,
+		GridSizeCM: f.GridSizeCM, SnapToGrid: f.SnapToGrid,
+	}
 }
 
 // gardenCreateRequest / gardenUpdateRequest are the JSON bodies. Update adds a

internal/api/objects.go

@@ -27,6 +27,8 @@ type objectCreateRequest struct {
 	Plantable   *bool           `json:"plantable"`
 	Color       *string         `json:"color"`
 	Props       json.RawMessage `json:"props"`
+	GridSizeCM  float64         `json:"gridSizeCm"`
+	SnapToGrid  bool            `json:"snapToGrid"`
 	Notes       string          `json:"notes"`
 }
 
@@ -35,7 +37,8 @@ func (r objectCreateRequest) toInput() service.ObjectInput {
 		Kind: r.Kind, Name: r.Name, Shape: r.Shape,
 		XCM: r.XCM, YCM: r.YCM, WidthCM: r.WidthCM, HeightCM: r.HeightCM,
 		RotationDeg: r.RotationDeg, ZIndex: r.ZIndex, Plantable: r.Plantable,
-		Color: r.Color, Props: propsFromRaw(r.Props), Notes: r.Notes,
+		Color: r.Color, Props: propsFromRaw(r.Props),
+		GridSizeCM: r.GridSizeCM, SnapToGrid: r.SnapToGrid, Notes: r.Notes,
 	}
 }
 
@@ -54,6 +57,8 @@ type objectUpdateRequest struct {
 	Plantable   *bool           `json:"plantable"`
 	Color       json.RawMessage `json:"color"`
 	Props       json.RawMessage `json:"props"`
+	GridSizeCM  *float64        `json:"gridSizeCm"`
+	SnapToGrid  *bool           `json:"snapToGrid"`
 	Notes       *string         `json:"notes"`
 	Version     int64           `json:"version" binding:"required,min=1"`
 }
@@ -67,7 +72,8 @@ func (r objectUpdateRequest) toPatch() (service.ObjectPatch, error) {
 	return service.ObjectPatch{
 		Name: r.Name, XCM: r.XCM, YCM: r.YCM, WidthCM: r.WidthCM, HeightCM: r.HeightCM,
 		RotationDeg: r.RotationDeg, ZIndex: r.ZIndex, Plantable: r.Plantable,
-		SetColor: setColor, Color: color, SetProps: setProps, Props: props, Notes: r.Notes,
+		SetColor: setColor, Color: color, SetProps: setProps, Props: props,
+		GridSizeCM: r.GridSizeCM, SnapToGrid: r.SnapToGrid, Notes: r.Notes,
 	}, nil
 }
 

internal/api/objects_test.go

@@ -25,6 +25,56 @@ func objectsPath(id int64) string {
 	return "/api/v1/gardens/" + strconv.FormatInt(id, 10) + "/objects"
 }
 
+// TestGridFieldsRoundTripAPI guards the JSON wiring (tags + toInput/toPatch) for
+// the garden and object grid/snap fields end-to-end through the HTTP handlers.
+func TestGridFieldsRoundTripAPI(t *testing.T) {
+	r := authEngine(t, localCfg())
+	cookie := registerAndCookie(t, r, "[email protected]")
+
+	// A garden created with no grid echoes the 1 m default, snapping off.
+	w := doJSON(t, r, http.MethodPost, "/api/v1/gardens", map[string]any{"name": "G"}, cookie)
+	if w.Code != http.StatusCreated {
+		t.Fatalf("create garden: status %d, body %s", w.Code, w.Body.String())
+	}
+	g := decodeMap(t, w.Body.Bytes())
+	gid := int64(g["id"].(float64))
+	if g["gridSizeCm"].(float64) != 100 || g["snapToGrid"].(bool) {
+		t.Errorf("garden grid defaults = %v / %v, want 100 / false", g["gridSizeCm"], g["snapToGrid"])
+	}
+
+	// Patching the garden's grid + snap echoes the new values.
+	w = doJSON(t, r, http.MethodPatch, gardenPath(gid), map[string]any{
+		"name": "G", "widthCm": 300, "heightCm": 200, "unitPref": "metric",
+		"gridSizeCm": 25, "snapToGrid": true, "version": 1,
+	}, cookie)
+	if w.Code != http.StatusOK {
+		t.Fatalf("patch garden: status %d, body %s", w.Code, w.Body.String())
+	}
+	if g := decodeMap(t, w.Body.Bytes()); g["gridSizeCm"].(float64) != 25 || !g["snapToGrid"].(bool) {
+		t.Errorf("patched garden grid = %v / %v, want 25 / true", g["gridSizeCm"], g["snapToGrid"])
+	}
+
+	// A bed created with no grid echoes the 30 cm default; a patch sets grid + snap.
+	w = doJSON(t, r, http.MethodPost, objectsPath(gid),
+		map[string]any{"kind": "bed", "xCm": 150, "yCm": 100, "widthCm": 120, "heightCm": 80}, cookie)
+	if w.Code != http.StatusCreated {
+		t.Fatalf("create object: status %d, body %s", w.Code, w.Body.String())
+	}
+	o := decodeMap(t, w.Body.Bytes())
+	oid := int64(o["id"].(float64))
+	if o["gridSizeCm"].(float64) != 30 || o["snapToGrid"].(bool) {
+		t.Errorf("object grid defaults = %v / %v, want 30 / false", o["gridSizeCm"], o["snapToGrid"])
+	}
+	w = doJSON(t, r, http.MethodPatch, objectPath(oid),
+		map[string]any{"gridSizeCm": 15, "snapToGrid": true, "version": 1}, cookie)
+	if w.Code != http.StatusOK {
+		t.Fatalf("patch object: status %d, body %s", w.Code, w.Body.String())
+	}
+	if o := decodeMap(t, w.Body.Bytes()); o["gridSizeCm"].(float64) != 15 || !o["snapToGrid"].(bool) {
+		t.Errorf("patched object grid = %v / %v, want 15 / true", o["gridSizeCm"], o["snapToGrid"])
+	}
+}
+
 func TestObjectCRUDAndFull(t *testing.T) {
 	r := authEngine(t, localCfg())
 	cookie := registerAndCookie(t, r, "[email protected]")