fix(a11y): render the kanban bucket footer as a list item

The add-task footer renders inside ul.tasks via the draggable footer
slot; as a div it was the one remaining invalid child of the kanban
lists converted in #3234.

Claude-Session: https://claude.ai/code/session_018prq5WqRL38EGhPNQKqZUP

frontend/src/components/project/views/ProjectKanban.vue

@@ -164,7 +164,7 @@
 									@end="updateTaskPosition"
 								>
 									<template #footer>
-										<div
+										<li
 											v-if="canCreateTasks"
 											class="bucket-footer"
 										>
@@ -210,7 +210,7 @@
 													bucket.tasks.length === 0 ? $t('project.kanban.addTask') : $t('project.kanban.addAnotherTask')
 												}}
 											</XButton>
-										</div>
+										</li>
 									</template>
 
 									<template #item="{element: task}">