submit add child form on enter key

extract the save-child handler into a submit closure shared by
the save button click and a keydown listener on the input. also
focus the input as soon as the form opens so the user can type
and hit enter without touching the mouse.

public/js/text.js

@@ -117,7 +117,8 @@ function toggleAddForm(li, parentNodeId, textId) {
     const saveBtn = document.createElement('button');
     saveBtn.textContent = 'Save';
     saveBtn.className = 'save-child';
-    saveBtn.addEventListener('click', () => {
+
+    function submit() {
         const title = input.value.trim();
         if (!title) return;
 
@@ -133,10 +134,19 @@ function toggleAddForm(li, parentNodeId, textId) {
                 return res.json();
             })
             .then(() => fetchAndRenderNodes(textId));
+    }
+
+    saveBtn.addEventListener('click', submit);
+    input.addEventListener('keydown', (event) => {
+        if (event.key === 'Enter') {
+            event.preventDefault();
+            submit();
+        }
     });
 
     li.appendChild(input);
     li.appendChild(saveBtn);
+    input.focus();
 }
 
 function toggleBulkAddForm(li, parentNodeId, textId) {