Add iOS app icon entry to Ghostty icon set

The icon set only had mac idiom entries, causing the iOS build to
fail with "did not have any applicable content". Add a universal
iOS entry reusing the 1024x1024 icon (512x512@2x).

Author: nicosuave

macos/Assets.xcassets/Ghostty.appiconset/Contents.json

@@ -59,6 +59,12 @@
       "size": "512x512",
       "scale": "2x",
       "filename": "icon_512x512@2x.png"
+    },
+    {
+      "idiom": "universal",
+      "platform": "ios",
+      "size": "1024x1024",
+      "filename": "icon_512x512@2x.png"
     }
   ],
   "info": {