fix(gantt): isolate chart stacking context so date picker renders above it

Sets position: relative and z-index: 0 on .gantt-chart-container to
create a new stacking context. This ensures z-index values inside the
Gantt chart (e.g. the sticky timeline header) cannot compete with the
Flatpickr popup that renders at the body level.

Ref #2337

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

@@ -162,6 +162,8 @@ const flatPickerConfig = computed(() => ({
 <style lang="scss" scoped>
 .gantt-chart-container {
 	padding-block-end: 1rem;
+	position: relative;
+	z-index: 0;
 }
 
 .gantt-options {