feat(gootower): replace sky drawing with background image asset

- Add gootower-bg to asset manifest
- Replace drawSky() with drawBackground() method that renders the background texture
- Display background when entering play mode

src/data/assetManifest.js

@@ -61,6 +61,9 @@ const sheet = (key, path, frameWidth, frameHeight) => ({ type: 'spritesheet', ke
 const image = (key, path) => ({ type: 'image', key, path });
 
 export const MANIFEST = {
+  gootower: [
+    image('gootower-bg', 'assets/images/gootower/background-1.png'),
+  ],
   forbiddenisland: [
     // Tiles: 2 cols (dry, flooded) × 24 rows. Row i → dry frame 2i, flooded
     // frame 2i+1 (see IslandData.TILE_FRAME_ROW).

src/games/gootower/GooTowerGame.js

@@ -111,8 +111,6 @@ export default class GooTowerGame extends Phaser.Scene {
       if (music?.tracks) new MusicPlayer(this, music.tracks);
     } catch (_) { /* optional */ }
 
-    this.drawSky();
-
     const raw = this.cache.json.get('gootower-levels');
     this.manifest = (raw?.levels ?? []).slice().sort((a, b) => a.level - b.level);
     this.chapters = raw?.chapters?.length
@@ -139,6 +137,13 @@ export default class GooTowerGame extends Phaser.Scene {
     g.fillRect(0, 0, GAME_WIDTH, GAME_HEIGHT);
   }
 
+  drawBackground() {
+    if (this.textures.exists('gootower-bg')) {
+      this.add.image(GAME_WIDTH / 2, GAME_HEIGHT / 2, 'gootower-bg')
+        .setDisplaySize(GAME_WIDTH, GAME_HEIGHT).setDepth(D.sky);
+    }
+  }
+
   clearBoard() {
     for (const o of this.boardObjs) o.destroy();
     this.boardObjs = [];
@@ -308,6 +313,7 @@ export default class GooTowerGame extends Phaser.Scene {
     if (!def) { this.showSelect(); return; }
 
     this.clearBoard();
+    this.drawBackground();
     this.viewMode = 'play';
     this.level = level;
     this.levelDef = def;