Image picker: tiles are containers, not graphics with children

The twin of the font picker fixed earlier today: every tile was a Graphics
carrying its sprite and caption, which Pixi v8 answers with "addChild:
Only Containers will be allowed to add children". The background moves
into its own Graphics inside a Container, and the step watches the console
while the picker opens, as the font one does.

Editor suite: 784 checks.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/editor/texture/ImagePicker.ts

@@ -1,4 +1,4 @@
-import { type FederatedPointerEvent, Graphics, Sprite, Text } from 'pixi.js';
+import { Container, type FederatedPointerEvent, Graphics, Sprite, Text } from 'pixi.js';
 import type { StoredTexture } from '../../core/store/TextureStore';
 import {
   Button,
@@ -37,7 +37,7 @@ interface PickerItem extends StoredTexture {
  */
 class ImageGrid extends Widget {
   private items: PickerItem[] = [];
-  private tiles: { box: Graphics; sprite: Sprite; caption: Text }[] = [];
+  private tiles: { box: Container; background: Graphics; sprite: Sprite; caption: Text }[] = [];
   private metrics: GridMetrics = gridMetrics(GRID_WIDTH, 0);
   private selectedFile: string | null = null;
   private readonly empty = new Text({ text: '', style: textStyle('muted') });
@@ -57,12 +57,15 @@ class ImageGrid extends Widget {
     this.items = items;
     for (const tile of this.tiles) tile.box.destroy({ children: true });
     this.tiles = items.map(() => {
-      const box = new Graphics();
+      // A Container holding the background, not a Graphics with children: Pixi v8 only lets
+      // containers have them (`addChild: Only Containers will be allowed to add children`).
+      const box = new Container();
+      const background = new Graphics();
       const sprite = new Sprite();
       const caption = new Text({ style: textStyle('small') });
-      box.addChild(sprite, caption);
+      box.addChild(background, sprite, caption);
       this.addChild(box);
-      return { box, sprite, caption };
+      return { box, background, sprite, caption };
     });
     if (!items.some((item) => item.file === this.selectedFile)) this.selectedFile = null;
     this.layout();
@@ -128,8 +131,8 @@ class ImageGrid extends Widget {
       const item = this.items[index];
       if (!item) return;
       const selected = item.file === this.selectedFile;
-      node.box.clear();
-      drawControlBox(node.box, side, side, {
+      node.background.clear();
+      drawControlBox(node.background, side, side, {
         fill: selected ? Theme.colors.selection : Theme.colors.control,
         border: selected ? Theme.colors.accent : Theme.colors.border,
       });