fix(ci): revert to universal macOS build with single notarization

- Replace separate ARM64 and x64 builds with single universal build
- Use --universal flag for fat binary (both archs in one app)
- Update package.json mac target to use "universal" arch
- Single notarization instead of double (fixes duplicate notarize calls)
- Simplify workflow by removing separate macOS release jobs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -121,15 +121,13 @@
         {
           "target": "dmg",
           "arch": [
-            "arm64",
-            "x64"
+            "universal"
           ]
         },
         {
           "target": "zip",
           "arch": [
-            "arm64",
-            "x64"
+            "universal"
           ]
         }
       ],