fix(a11y): label the bucket selector with its purpose and current bucket

The trigger announced only the bare bucket name ("To-Do, button");
it now announces "Kanban bucket: To-Do".

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

frontend/src/components/tasks/partials/BucketSelect.vue

@@ -7,6 +7,7 @@
 					<BaseButton
 						class="bucket-name"
 						:aria-expanded="open"
+						:aria-label="$t('task.detail.bucketSelectLabel', {bucket: currentBucketTitle})"
 						@click="toggleOpen"
 					>
 						{{ currentBucketTitle }}

frontend/src/i18n/lang/en.json

@@ -994,6 +994,7 @@
       "deleteSuccess": "The task has been deleted successfully.",
       "duplicateSuccess": "The task was duplicated successfully.",
       "noBucket": "No bucket",
+      "bucketSelectLabel": "Kanban bucket: {bucket}",
       "bucketChangedSuccess": "The task bucket has been changed successfully.",
       "belongsToProject": "This task belongs to project '{project}'",
       "back": "Back to project",