chore: prepare the 1.3.0 release

Version 1.3.0 (manifest "1.3"): the options page rewrite adds features
without breaking changes, and addons.mozilla.org needs a version above
the live 1.2. The lockfile's root version moves with it. Unreleased
becomes the 1.3.0 section.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

manifest.json

@@ -4,7 +4,7 @@
   "manifest_version": 2,
   "author": "Mathew Guest <mat@zavage.net>",
   "homepage_url": "https://zavage-software.com/portfolio/search-with-ChatGPT",
-  "version": "1.2",
+  "version": "1.3",
   "icons": {
     "16": "icons/powered-by-openai-icon-16x16.png",
     "32": "icons/powered-by-openai-icon-32x32.png",