fix: resolve electron-builder signing error on GitHub Actions

- Change identity from '-' to null to skip electron-builder signing
- Add CSC_FOR_PULL_REQUEST env var to ensure signing is skipped
- Keep manual ad-hoc signing step after build for consistency

This fixes the 'no valid identity' error during the build process.

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

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

Author: ishikawa-pro

.github/workflows/build-and-release.yml

@@ -18,6 +18,7 @@ jobs:
 
     env:
       CSC_IDENTITY_AUTO_DISCOVERY: "false"
+      CSC_FOR_PULL_REQUEST: "true"
 
     steps:
       - name: Checkout code

electron-builder.json

@@ -30,7 +30,7 @@
       }
     ],
     "icon": "logo.png",
-    "identity": "-",
+    "identity": null,
     "hardenedRuntime": true,
     "gatekeeperAssess": false,
     "artifactName": "${productName}-${version}-${os}-${arch}.${ext}"

package.json

@@ -1,6 +1,6 @@
 {
   "name": "diff-viewer",
-  "version": "0.1.14",
+  "version": "0.1.15",
   "description": "A rich Git diff viewer with syntax highlighting",
   "main": "build/electron/main.js",
   "homepage": "./",