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