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/service/gardens.go

@@ -13,11 +13,12 @@ import (
 // sanity range that also rejects NaN/Inf and absurd values. Name/notes are
 // length-capped so untrusted input can't balloon storage.
 const (
-	defaultGardenCM   = 1000   // 10 m
-	minGardenCM       = 1      // 1 cm
-	maxGardenCM       = 10_000 // 100 m
-	maxGardenNameLen  = 200
-	maxGardenNotesLen = 10_000
+	defaultGardenCM     = 1000   // 10 m
+	minGardenCM         = 1      // 1 cm
+	maxGardenCM         = 10_000 // 100 m
+	defaultGardenGridCM = 100    // 1 m, matching the editor's previous fixed grid
+	maxGardenNameLen    = 200
+	maxGardenNotesLen   = 10_000
 )
 
 // gardenRole ranks a user's access to a garden. Higher includes lower
@@ -48,11 +49,13 @@ func (r gardenRole) String() string {
 
 // GardenInput is the mutable field set for creating or updating a garden.
 type GardenInput struct {
-	Name     string
-	WidthCM  float64
-	HeightCM float64
-	UnitPref string
-	Notes    string
+	Name       string
+	WidthCM    float64
+	HeightCM   float64
+	UnitPref   string
+	Notes      string
+	GridSizeCM float64
+	SnapToGrid bool
 }
 
 // requireGardenRole loads a garden and enforces that the actor holds at least
@@ -198,12 +201,26 @@ func gardenFromInput(in GardenInput, applyDefaults bool) (*domain.Garden, error)
 		return nil, domain.ErrInvalidInput
 	}
 
+	// Grid spacing shares the garden dimension range [1 cm, 100 m]. 0 means
+	// "unset" — defaulted (not just on create: an older client that omits it must
+	// not wipe the column to an invalid 0), so a saved garden always has a usable
+	// grid even if snapping is off.
+	grid := in.GridSizeCM
+	if grid == 0 {
+		grid = defaultGardenGridCM
+	}
+	if !validDimensionCM(grid) {
+		return nil, domain.ErrInvalidInput
+	}
+
 	return &domain.Garden{
-		Name:     name,
-		WidthCM:  width,
-		HeightCM: height,
-		UnitPref: unit,
-		Notes:    notes,
+		Name:       name,
+		WidthCM:    width,
+		HeightCM:   height,
+		UnitPref:   unit,
+		Notes:      notes,
+		GridSizeCM: grid,
+		SnapToGrid: in.SnapToGrid,
 	}, nil
 }