Fix New Document button and make editor full-screen

- Replace prompt() with inline title form (button → input → Create/Cancel)
- Editor view: position:fixed inset:0 z-index:200 so it covers full screen
- Docs editor toolbar moved into CSS (was duplicated in styles)

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

public/app.js

@@ -2603,11 +2603,25 @@ async function openDocEditor(id, key, title) {
   }
 }
 
-$('#btn-new-doc').addEventListener('click', async () => {
-  const title = prompt('Document title:', 'Untitled Document');
-  if (!title) return;
+$('#btn-new-doc').addEventListener('click', () => {
+  $('#btn-new-doc').hidden = true;
+  $('#docs-new-form').hidden = false;
+  $('#docs-new-title').value = '';
+  $('#docs-new-title').focus();
+});
+
+$('#btn-docs-new-cancel').addEventListener('click', () => {
+  $('#docs-new-form').hidden = true;
+  $('#btn-new-doc').hidden = false;
+});
+
+$('#docs-new-form').addEventListener('submit', async (e) => {
+  e.preventDefault();
+  const title = $('#docs-new-title').value.trim() || 'Untitled Document';
+  $('#docs-new-form').hidden = true;
+  $('#btn-new-doc').hidden = false;
   const { id, key } = await api('/api/docs', { method: 'POST', body: { studentId: currentStudent.id, title } });
-  loadDocs();
+  await loadDocs();
   openDocEditor(id, key, title);
 });