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

web/src/editor/SelectionOverlay.tsx

@@ -1,5 +1,5 @@
 import { useEffect, useRef, type PointerEvent as ReactPointerEvent, type RefObject } from 'react'
-import { localToWorld, screenToWorld, worldToLocal, type Point } from '@/lib/geometry'
+import { localToWorld, screenToWorld, snapPoint, snapValue, worldToLocal, type Point } from '@/lib/geometry'
 import { useUpdateObject } from '@/lib/objects'
 import { HANDLE_PX, SELECT_COLOR, objectTransform } from './shared'
 import { useEditorStore } from './store'
@@ -27,10 +27,16 @@ export function SelectionOverlay({
   object,
   gardenId,
   svgRef,
+  snap,
+  gridCm,
 }: {
   object: EditorObject
   gardenId: number
   svgRef: RefObject<SVGSVGElement | null>
+  // The garden grid: when snap is true, a move snaps the object's center to it and
+  // a resize snaps width/height to whole grid steps (the opposite corner stays put).
+  snap: boolean
+  gridCm: number
 }) {
   const setLiveObject = useEditorStore((s) => s.setLiveObject)
   const setObjectDragging = useEditorStore((s) => s.setObjectDragging)
@@ -112,7 +118,9 @@ export function SelectionOverlay({
       e,
       (ev) => {
         const world = pointerWorld(ev)
-        return { ...base, xCm: base.xCm + (world.x - start.x), yCm: base.yCm + (world.y - start.y) }
+        const next: Point = { x: base.xCm + (world.x - start.x), y: base.yCm + (world.y - start.y) }
+        const c = snap ? snapPoint(next, gridCm) : next
+        return { ...base, xCm: c.x, yCm: c.y }
       },
       (f) => ({ id: base.id, version: base.version, xCm: f.xCm, yCm: f.yCm }),
     )
@@ -128,8 +136,15 @@ export function SelectionOverlay({
       (ev) => {
         const world = pointerWorld(ev)
         const p = worldToLocal(world, center0, base.rotationDeg)
-        const newW = Math.max(MIN_OBJ_CM, Math.abs(p.x - oppositeLocal.x))
-        const newH = Math.max(MIN_OBJ_CM, Math.abs(p.y - oppositeLocal.y))
+        let newW = Math.max(MIN_OBJ_CM, Math.abs(p.x - oppositeLocal.x))
+        let newH = Math.max(MIN_OBJ_CM, Math.abs(p.y - oppositeLocal.y))
+        // Snap dimensions to whole grid steps (at least one cell). The opposite
+        // corner is the anchor, so it stays fixed while the dragged corner lands
+        // on a grid-multiple size.
+        if (snap) {
+          newW = Math.max(gridCm, snapValue(newW, gridCm))
+          newH = Math.max(gridCm, snapValue(newH, gridCm))
+        }
         const draggedLocal: Point = { x: oppositeLocal.x + sx * newW, y: oppositeLocal.y + sy * newH }
         const newCenterLocal: Point = {
           x: (oppositeLocal.x + draggedLocal.x) / 2,