test(store): add 13-inch iPad screenshots (universal app requires them)

App ships universal (iPhone+iPad), so the App Store requires iPad
screenshots. Render the 3 store screens at 2048x2732 (iPad 12.9"/13").

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/tool/generate_store_screenshots_test.dart

@@ -30,6 +30,7 @@ typedef _Device = ({String dir, int pw, int ph, double dpr});
 const _devices = <_Device>[
   (dir: 'ios', pw: 1290, ph: 2796, dpr: 3.0), // iPhone 6.7"
   (dir: 'android', pw: 1080, ph: 1920, dpr: 3.0), // Play phone
+  (dir: 'ipad', pw: 2048, ph: 2732, dpr: 2.0), // iPad 12.9"/13"
 ];
 
 // A lively mid-game board: three gems to clear plus scattered colored blocks.