fix: remove invalid afterSign configuration from electron-builder

The afterSign property expects a function, not a boolean value.
Removing it to fix the build error.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: ishikawa-pro

electron-builder.json

@@ -16,7 +16,6 @@
   "npmRebuild": false,
   "buildDependenciesFromSource": true,
   "nodeGypRebuild": false,
-  "afterSign": false,
   "mac": {
     "category": "public.app-category.developer-tools",
     "target": [