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/migrations/0005_fix_category_verbose_name_plural.py

@@ -0,0 +1,17 @@
+# Generated by Django 5.2.12 on 2026-03-19 00:10
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('blog', '0004_backfill_published_date'),
+    ]
+
+    operations = [
+        migrations.AlterModelOptions(
+            name='category',
+            options={'ordering': ['sort_order', 'name'], 'verbose_name_plural': 'categories'},
+        ),
+    ]