refactor: implement inventory item spawner and config

- Add cell and text styling constants to `GRID_CONFIG`
- Replace manual item drawing in `InventoryTestScene` with
  `InventoryItemSpawner`
- Update `InventoryTestScene` to use dynamic inventory dimensions for
  grid rendering
- Register `InventoryTestScene` in the main `App` component

packages/sts-like-viewer/src/ui/App.tsx

@@ -4,6 +4,7 @@ import { MapViewerScene } from "@/scenes/MapViewerScene";
 import { GridViewerScene } from "@/scenes/GridViewerScene";
 import { ShapeViewerScene } from "@/scenes/ShapeViewerScene";
 import { GAME_CONFIG } from "@/config";
+import { InventoryTestScene } from "@/scenes/InventoryTestScene";
 
 export default function App() {
   return (
@@ -11,6 +12,7 @@ export default function App() {
       <div className="flex-1 flex relative justify-center items-center">
         <PhaserGame initialScene="IndexScene" config={GAME_CONFIG}>
           <PhaserScene scene={IndexScene} />
+          <PhaserScene scene={InventoryTestScene} />
           <PhaserScene scene={MapViewerScene} />
           <PhaserScene scene={GridViewerScene} />
           <PhaserScene scene={ShapeViewerScene} />