refactor: update templates for new frontend structure

- Extract inline JavaScript from index.templ
- Replace with external script include for theme.js
- All templates reference /static/ for assets
- Cleaner separation of concerns between markup and logic

templates/admin.templ

@@ -7,6 +7,7 @@ templ Admin(user User) {
         <meta charset="UTF-8">
         <title>Admin Dashboard - Bookmann</title>
         <script src="/static/htmx.min.js"></script>
+        <script src="/static/toast.js"></script>
         <link href="/static/style.css" rel="stylesheet">
         <style>
             :root {