feat: add macOS code signing and notarization support

- Add entitlements.mac.plist for hardened runtime
- Add notarize.js post-sign hook for Apple notarization
- Update package.json with signing config and @electron/notarize dep
- Update GitHub Actions workflow with signing secrets

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

.github/workflows/release.yml

@@ -40,6 +40,14 @@ jobs:
       - run: npm ci
 
       - name: Build macOS Packages
+        env:
+          # Code signing
+          CSC_LINK: ${{ secrets.CSC_LINK }}
+          CSC_KEY_PASSWORD: ${{ secrets.CSC_KEY_PASSWORD }}
+          # Notarization
+          APPLE_ID: ${{ secrets.APPLE_ID }}
+          APPLE_APP_SPECIFIC_PASSWORD: ${{ secrets.APPLE_APP_SPECIFIC_PASSWORD }}
+          APPLE_TEAM_ID: ${{ secrets.APPLE_TEAM_ID }}
         run: npx electron-builder --mac --publish never
       - uses: actions/upload-artifact@v4
         with: