feat(macos): add code signing and notarization support

Add macOS code signing and notarization for Gatekeeper compatibility:

- Add hardened runtime configuration in package.json
- Add entitlements.mac.plist for required app permissions
- Enable built-in electron-builder notarization
- Add code signing and notarization secrets to workflow

Required GitHub Secrets:
- CSC_LINK: Base64-encoded .p12 certificate file
- CSC_KEY_PASSWORD: Password for the .p12 certificate
- APPLE_ID: Apple Developer account email
- APPLE_APP_SPECIFIC_PASSWORD: App-specific password from appleid.apple.com
- APPLE_TEAM_ID: 10-character Apple Developer Team ID

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

build/entitlements.mac.plist

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <key>com.apple.security.cs.allow-jit</key>
+    <true/>
+    <key>com.apple.security.cs.allow-unsigned-executable-memory</key>
+    <true/>
+    <key>com.apple.security.cs.disable-library-validation</key>
+    <true/>
+    <key>com.apple.security.network.client</key>
+    <true/>
+    <key>com.apple.security.network.server</key>
+    <true/>
+    <key>com.apple.security.files.user-selected.read-write</key>
+    <true/>
+</dict>
+</plist>