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>

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>