refactor(onitama): centralize visual constants in config

Introduce `TEXT_POSITION` and `VISUAL` objects to the configuration
to manage magic numbers for positioning, radii, stroke widths, and
alphas. Update renderers and spawners to use these constants instead
of hardcoded values or direct calculations.

packages/sts-like-viewer/src/scenes/ShapeViewerScene.ts

@@ -17,6 +17,19 @@ export class ShapeViewerScene extends ReactiveScene {
     super.create();
     this.drawShapeViewer();
     this.createControls();
+    this.createBackButton();
+  }
+
+  private createBackButton(): void {
+    createButton({
+      scene: this,
+      label: "← Back",
+      x: 80,
+      y: 30,
+      onClick: async () => {
+        await this.sceneController.launch(SceneKey.IndexScene);
+      },
+    });
   }
 
   private drawShapeViewer(): void {