fix: remove conflicting forceCodeSigning option

The forceCodeSigning: true option conflicts with identity: null.
Removed it to allow electron-builder to skip signing as intended.

🤖 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,
-  "forceCodeSigning": true,
   "mac": {
     "category": "public.app-category.developer-tools",
     "target": [

package.json

@@ -1,6 +1,6 @@
 {
   "name": "diff-viewer",
-  "version": "0.1.15",
+  "version": "0.1.16",
   "description": "A rich Git diff viewer with syntax highlighting",
   "main": "build/electron/main.js",
   "homepage": "./",