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/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]")