Add hierarchical tasks with tree and flat mode

- parent_id on tasks; roots shown by default, others expandable
- Expand/collapse per node with expand/collapse button
- Sub button creates subtask directly from the row
- Make subtask of button in edit dialog with searchable picker
- Remove parent clears the parent link in the dialog
- Flat mode toggle shows all tasks indented by depth
- Tree filter shows nodes whose descendants match, not only direct matches
- Deletion blocked if task has subtasks (client + server)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/storage.mjs

@@ -32,3 +32,12 @@ export function set_task(task) {
 export function delete_task(id) {
 	return store.delete(`task:${id}`);
 }
+
+export function has_child_tasks(parent_id) {
+	for (const [key] of store.data.entries()) {
+		if (!key.startsWith('task:')) { continue; }
+		const task = store.get(key);
+		if (task.parent_id === parent_id) { return true; }
+	}
+	return false;
+}