feat: add delete button to task detail and edit pages

Task detail page gets a Delete button in the header bar (matching
all other entity detail pages). Task edit form gets a Delete button
below the form. Both use confirmation dialog via data-confirm.

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

templates/task_form.html

@@ -116,4 +116,11 @@
         </div>
     </form>
 </div>
+{% if item %}
+<div style="margin-top:12px;text-align:right;">
+    <form action="/tasks/{{ item.id }}/delete" method="post" data-confirm="Delete this task?" style="display:inline">
+        <button type="submit" class="btn btn-danger btn-sm">Delete Task</button>
+    </form>
+</div>
+{% endif %}
 {% endblock %}