fix(kanban): prevent task taps from leaking through the sticky add-task footer on touch devices

The sticky bucket footer had no z-index, so the absolutely positioned
`.handle` overlays on each task (z-index: 1, used to capture taps on
touch devices) stacked above the Add Task button. Tapping the button
where a task scrolled behind it would open that task instead of opening
the new-task input.

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

@@ -1094,6 +1094,7 @@ $filter-container-height: '1rem - #{$switch-view-height}';
 	.bucket-footer {
 		position: sticky;
 		inset-block-end: 0;
+		z-index: 2;
 		block-size: min-content;
 		padding: .5rem;
 		background-color: var(--grey-100);