feat(scene): battle grid overlay with color and trap VFX stacking

Add configurable square/hex grid in the scene editor (persisted and shown on control/presentation) and keep trap activation effects above trap icons.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/shared/ipc/contracts.ts

@@ -20,6 +20,7 @@ import type {
   Scene,
   SceneDarknessEvent,
   SceneDarknessState,
+  SceneGrid,
   SceneId,
   SceneTrap,
   SceneTrapsEvent,
@@ -703,6 +704,7 @@ export type ScenePatch = {
   previewRotationDeg?: 0 | 90 | 180 | 270;
   darkenScene?: boolean;
   traps?: SceneTrap[];
+  grid?: SceneGrid;
   settings?: Partial<Scene['settings']>;
   media?: Partial<Scene['media']>;
   layout?: Partial<Scene['layout']>;