Add custom background artwork to Super Kart menu screens

- Add superkart-background.png image asset
- Replace solid-color rectangle background with artwork in menu views
- Add semi-transparent shade overlay for better contrast
- Remove hardcoded title and tagline text (now baked into the image)

src/data/assetManifest.js

@@ -172,6 +172,7 @@ export const MANIFEST = {
   // entries stay procedural (tinted kart silhouettes, painted horizon).
   superkart: [
     { type: 'json', key: 'superkart-cups', path: 'assets/gamedata/superkart/cups.json' },
+    image('superkart-background', 'assets/images/superkart-background.png'),
     (scene) => sheetsFrom(scene, 'superkart-artwork', ['racerSheets', 'themeSheets', 'itemSheet']),
     (scene) => {
       const art = scene.cache.json.get('superkart-artwork');

src/games/superkart/SuperKartGame.js

@@ -77,8 +77,18 @@ export default class SuperKartGame extends Phaser.Scene {
       this.teardownRace();
     });
 
-    this.bg = this.add.rectangle(GAME_WIDTH / 2, GAME_HEIGHT / 2, GAME_WIDTH, GAME_HEIGHT, 0x101018)
-      .setDepth(-10);
+    // Menu-screen art: covers the whole canvas but is itself covered by the
+    // Mode 7 ground/backdrop (depths -1/-2) during a race, so it only shows
+    // through on the menu / select / results / podium views.
+    if (this.textures.exists('superkart-background')) {
+      this.bg = this.add.image(GAME_WIDTH / 2, GAME_HEIGHT / 2, 'superkart-background')
+        .setDisplaySize(GAME_WIDTH, GAME_HEIGHT).setDepth(-10);
+    } else {
+      this.bg = this.add.rectangle(GAME_WIDTH / 2, GAME_HEIGHT / 2, GAME_WIDTH, GAME_HEIGHT, 0x101018)
+        .setDepth(-10);
+    }
+    this.bgShade = this.add.rectangle(GAME_WIDTH / 2, GAME_HEIGHT / 2, GAME_WIDTH, GAME_HEIGHT, 0x0a0a12, 0.45)
+      .setDepth(-9);
 
     if (this.renderer.type !== Phaser.WEBGL) {
       this.showWebGLNotice();
@@ -248,8 +258,7 @@ export default class SuperKartGame extends Phaser.Scene {
     this.teardownRace();
     this.clearView();
     const cx = GAME_WIDTH / 2;
-    this.text(cx, 200, 'SUPER KART', 110, '#ffd028');
-    this.text(cx, 290, 'MODE 7 KART RACING  •  STARRING THE FERTIG GAMES CREW', 24, COLORS.mutedHex);
+    // Title/tagline are baked into superkart-background.png — no text needed here.
     this.text(cx, 380,
       'ARROWS / WASD DRIVE  •  SPACE HOP+DRIFT  •  SHIFT OR X FIRES ITEMS', 22, COLORS.mutedHex);