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>

scripts/notarize.js

@@ -0,0 +1,36 @@
+const { notarize } = require('@electron/notarize');
+const path = require('path');
+
+exports.default = async function notarizing(context) {
+  const { electronPlatformName, appOutDir } = context;
+
+  // Only notarize macOS builds
+  if (electronPlatformName !== 'darwin') {
+    console.log('Skipping notarization: not macOS');
+    return;
+  }
+
+  // Skip if credentials not provided (local builds)
+  if (!process.env.APPLE_ID || !process.env.APPLE_APP_SPECIFIC_PASSWORD || !process.env.APPLE_TEAM_ID) {
+    console.log('Skipping notarization: missing credentials (APPLE_ID, APPLE_APP_SPECIFIC_PASSWORD, or APPLE_TEAM_ID)');
+    return;
+  }
+
+  const appName = context.packager.appInfo.productFilename;
+  const appPath = path.join(appOutDir, `${appName}.app`);
+
+  console.log(`Notarizing ${appPath}...`);
+
+  try {
+    await notarize({
+      appPath,
+      appleId: process.env.APPLE_ID,
+      appleIdPassword: process.env.APPLE_APP_SPECIFIC_PASSWORD,
+      teamId: process.env.APPLE_TEAM_ID,
+    });
+    console.log('Notarization complete!');
+  } catch (error) {
+    console.error('Notarization failed:', error);
+    throw error;
+  }
+};