Add free-type spell checker for students

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

server.js

@@ -7,6 +7,8 @@ import { readFile, writeFile, unlink } from 'node:fs/promises';
 import { createHash, createHmac, randomUUID } from 'node:crypto';
 import mammoth from 'mammoth';
 import multer from 'multer';
+import nspell from 'nspell';
+import dictionaryEn from 'dictionary-en';
 import { db, sha256 } from './db.js';
 
 const ROOT = dirname(fileURLToPath(import.meta.url));
@@ -435,6 +437,16 @@ app.get('/api/state/:studentId', (req, res) => {
   res.json({ student: { ...student, isBirthdayWeek: isBirthdayWeek(student.birthday) }, assignment, listProgress });
 });
 
+// Free-type spell check: any word, checked against a bundled English dictionary
+const speller = nspell(dictionaryEn);
+app.get('/api/spellcheck', (req, res) => {
+  const word = String(req.query.word || '').trim();
+  if (!/^[A-Za-z'’-]{1,60}$/.test(word)) return res.status(400).json({ error: 'Type a single word using letters only.' });
+  const clean = word.replace(/’/g, "'");
+  const correct = speller.correct(clean) || speller.correct(clean.toLowerCase());
+  res.json({ word, correct, suggestions: correct ? [] : speller.suggest(clean).slice(0, 4) });
+});
+
 // Build a practice session: the target list's unmastered words only.
 // listId overrides the student's pinned weekly assignment (used when launched from a course item).
 app.get('/api/session/:studentId', (req, res) => {