feat: add task and habit reordering

frontend/src/lib/task-utils.ts

@@ -88,6 +88,22 @@ export function groupTaskTree<T extends MinimalTask>(tasks: T[]) {
   return tasks.filter((task) => !task.parent_id).map((task) => ({ task, subtasks: children.get(task.id) ?? [] }))
 }
 
+export function moveItemWithinScope<T extends { id: string; parent_id?: string | null }>(items: T[], sourceId: string, targetId: string, placement: 'before' | 'after') {
+  if (sourceId === targetId) return items
+  const source = items.find((item) => item.id === sourceId)
+  const target = items.find((item) => item.id === targetId)
+  if (!source || !target || (source.parent_id ?? null) !== (target.parent_id ?? null)) return items
+  const scoped = items.filter((item) => (item.parent_id ?? null) === (source.parent_id ?? null))
+  const sourceIndex = scoped.findIndex((item) => item.id === sourceId)
+  const targetIndex = scoped.findIndex((item) => item.id === targetId)
+  const reordered = scoped.filter((item) => item.id !== sourceId)
+  const adjustedTargetIndex = reordered.findIndex((item) => item.id === targetId)
+  reordered.splice(adjustedTargetIndex + (placement === 'after' ? 1 : 0), 0, source)
+  if (sourceIndex === targetIndex || reordered.every((item, index) => item.id === scoped[index]?.id)) return items
+  const reorderedIterator = reordered[Symbol.iterator]()
+  return items.map((item) => (item.parent_id ?? null) === (source.parent_id ?? null) ? reorderedIterator.next().value! : item)
+}
+
 export function toDateTimeLocal(value: string | null | undefined) {
   if (!value) return ''
   const date = new Date(value)