Fix admin message auto-dismiss and Category plural label

Admin messages now auto-clear after 8 seconds via the w-messages
Stimulus controller's autoClear value, preventing message pile-up.
Category model gains verbose_name_plural so Wagtail shows "Categories"
instead of "Categorys" in the snippets menu.

Closes #62

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/wagtailadmin/base.html

@@ -8,7 +8,7 @@
     {% keyboard_shortcuts_dialog %}
     <main class="content-wrapper w-overflow-x-hidden" id="main">
         <div class="content">
-            <div class="messages" role="status" data-controller="w-messages" data-action="w-messages:add@document->w-messages#add" data-w-messages-added-class="new" data-w-messages-show-class="appear" data-w-messages-show-delay-value="100">
+            <div class="messages" role="status" data-controller="w-messages" data-action="w-messages:add@document->w-messages#add" data-w-messages-added-class="new" data-w-messages-show-class="appear" data-w-messages-show-delay-value="100" data-w-messages-auto-clear-value="8000">
                 <ul data-w-messages-target="container">
                     {% if messages %}
                         {% for message in messages %}