chore: bump version to 1.3.1

- update manifest.base.json, package.json and index.html to v1.3.1

- add SECURITY.md file

extension/manifest.base.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "KoalaSync",
-  "version": "1.2.1",
+  "version": "1.3.1",
   "description": "Watch party extension to synchronize video playback on YouTube, Twitch, Netflix, and HTML5 sites in real-time with friends.",
   "permissions": [
     "storage",