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>

apps/blog/models.py

@@ -168,6 +168,7 @@ class Category(models.Model):
 
     class Meta:
         ordering = ["sort_order", "name"]
+        verbose_name_plural = "categories"
 
     def __str__(self):
         return self.name