Fix editor full-screen and add parent-side doc viewer

Student editor:
- Add body.docs-editor-open class when editor opens; hide hero, nav,
  confetti so nothing bleeds behind the fixed overlay

Admin docs panel:
- New "Documents" sidebar entry listing all submitted docs with student
  name, date, and unread badge
- Click any doc to open it in OnlyOffice in view (read-only) mode
- Download button for offline access
- /api/docs/bykey/:key/editor-config server route for admin viewer

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

public/app.js

@@ -2571,6 +2571,7 @@ async function openDocEditor(id, key, title) {
   $('#btn-docs-submit').disabled = false;
   $('#btn-docs-submit').textContent = 'Submit to Parent ✓';
 
+  document.body.classList.add('docs-editor-open');
   const frame = $('#docs-editor-frame');
   frame.innerHTML = '';
   if (docsEditorInstance) {
@@ -2626,6 +2627,7 @@ $('#docs-new-form').addEventListener('submit', async (e) => {
 });
 
 $('#btn-docs-back').addEventListener('click', () => {
+  document.body.classList.remove('docs-editor-open');
   if (docsEditorInstance) {
     try { docsEditorInstance.destroyEditor(); } catch { /* ignore */ }
     docsEditorInstance = null;