🤖 Enable DMG code signing

The .app bundles were being signed, but the final DMG files were not.
Adding dmg.sign configuration to ensure the DMG itself is also signed.

_Generated with `cmux`_

Author: ammario

package.json

@@ -117,6 +117,9 @@
       "entitlements": "build/entitlements.mac.plist",
       "entitlementsInherit": "build/entitlements.mac.plist"
     },
+    "dmg": {
+      "sign": true
+    },
     "linux": {
       "target": "AppImage",
       "category": "Development",