Add word list preview, young learner mode, and practice improvements

- Show full word list before practice starts so students can study words first
- Add per-student young learner mode (set in admin): word stays visible while typing
- Center all practice card content (speak button, prompt, feedback, check button)
- Remove 10-word-per-session cap so large lists (25-30 words) practice all words
- Double the misspelled-word study countdown from 4s to 8s
- Fix word list preview numbering for 10+ items (inline numbers, no clipping)
- Add young_learner column to students table with admin toggle

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

server.js

@@ -325,7 +325,7 @@ const GRADABLE_TYPES = ['assignment', 'quiz', 'matching', 'crossword', 'spelling
 // ---------- kid picker ----------
 
 app.get('/api/students', (req, res) => {
-  res.json(db.prepare(`SELECT id, name, emoji, theme, bg_pattern, streak_count, streak_date FROM students ORDER BY name`).all());
+  res.json(db.prepare(`SELECT id, name, emoji, theme, bg_pattern, streak_count, streak_date, young_learner FROM students ORDER BY name`).all());
 });
 
 const VALID_THEMES = ['blue','green','purple','orange','pink','red','teal','yellow','indigo'];
@@ -343,6 +343,12 @@ app.patch('/api/students/:id/bg-pattern', (req, res) => {
   res.json({ ok: true });
 });
 
+app.patch('/api/students/:id/young-learner', requirePin, (req, res) => {
+  const val = req.body.young_learner ? 1 : 0;
+  db.prepare(`UPDATE students SET young_learner = ? WHERE id = ?`).run(val, req.params.id);
+  res.json({ ok: true });
+});
+
 app.patch('/api/students/:id/emoji', (req, res) => {
   const emoji = req.body.emoji ? String(req.body.emoji).trim() : null;
   if (!emoji) return res.status(400).json({ error: 'emoji required' });
@@ -370,7 +376,7 @@ app.post('/api/students/:id/complete-day', (req, res) => {
 // Everything the standalone spelling home screen needs
 app.get('/api/state/:studentId', (req, res) => {
   const id = req.params.studentId;
-  const student = db.prepare(`SELECT id, name, emoji, theme, bg_pattern FROM students WHERE id = ?`).get(id);
+  const student = db.prepare(`SELECT id, name, emoji, theme, bg_pattern, young_learner FROM students WHERE id = ?`).get(id);
   if (!student) return res.status(404).json({ error: 'No such student' });
 
   const assignment = db.prepare(`
@@ -415,7 +421,6 @@ app.get('/api/session/:studentId', (req, res) => {
     LEFT JOIN progress p ON p.word_id = w.id AND p.student_id = ?
     WHERE w.list_id = ? AND COALESCE(p.box, 0) < ${MASTERED_BOX}
     ORDER BY box, RANDOM()
-    LIMIT 10
   `).all(id, listId);
 
   const reviews = db.prepare(`