preserve expanded state across node re-render

introduce a module-level expandedNodeIds set that tracks which
nodes the user has manually expanded. renderTree consults the set
when deciding initial visibility (falling back to the depth-based
default for a fresh load), the toggle click handler keeps the set
in sync, and both add-child save handlers add the parent's id
before triggering the re-fetch. on a fresh load the set starts
empty so root-only-open behavior is unchanged and the existing
toggle tests keep passing.

public/js/text.js

@@ -1,3 +1,5 @@
+const expandedNodeIds = new Set();
+
 document.addEventListener('DOMContentLoaded', () => {
     const textId = window.location.pathname.split('/').pop();
 
@@ -69,7 +71,11 @@ function renderTree(nodes, textId, depth = 0) {
 
         if (node.children.length > 0) {
             const childUl = renderTree(node.children, textId, depth + 1);
-            const childrenVisible = depth === 0;
+            const childrenVisible =
+                expandedNodeIds.has(node.id) || depth === 0;
+            if (childrenVisible) {
+                expandedNodeIds.add(node.id);
+            }
             childUl.style.display = childrenVisible ? '' : 'none';
 
             const toggleBtn = document.createElement('button');
@@ -79,6 +85,11 @@ function renderTree(nodes, textId, depth = 0) {
                 const isHidden = childUl.style.display === 'none';
                 childUl.style.display = isHidden ? '' : 'none';
                 toggleBtn.textContent = isHidden ? '▼' : '▶';
+                if (isHidden) {
+                    expandedNodeIds.add(node.id);
+                } else {
+                    expandedNodeIds.delete(node.id);
+                }
             });
 
             li.appendChild(toggleBtn);
@@ -110,6 +121,7 @@ function toggleAddForm(li, parentNodeId, textId) {
         const title = input.value.trim();
         if (!title) return;
 
+        expandedNodeIds.add(parentNodeId);
         fetch('/api/nodes', {
             method: 'POST',
             headers: { 'Content-Type': 'application/json' },
@@ -155,6 +167,7 @@ function toggleBulkAddForm(li, parentNodeId, textId) {
         const count = parseInt(countInput.value);
         if (!titlePrefix || !count || count < 1) return;
 
+        expandedNodeIds.add(parentNodeId);
         fetch('/api/nodes/bulk', {
             method: 'POST',
             headers: { 'Content-Type': 'application/json' },