chore: add UI styles and update documentation

- Add cart drawer and product gallery thumbnail CSS
- Remove redundant type="text/javascript" attribute
- Update image optimization plan to reflect completion

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

lib/simpleshop_theme_web/components/layouts/root.html.heex

@@ -8,7 +8,7 @@
       {assigns[:page_title]}
     </.live_title>
     <link phx-track-static rel="stylesheet" href={~p"/assets/css/app.css"} />
-    <script defer phx-track-static type="text/javascript" src={~p"/assets/js/app.js"}>
+    <script defer phx-track-static src={~p"/assets/js/app.js"}>
     </script>
     <script>
       (() => {