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>

public/index.html

@@ -195,6 +195,17 @@
     <div id="flash-done" class="done-card" hidden></div>
   </section>
 
+  <!-- Word list preview (shown before practice starts) -->
+  <section id="view-word-preview" class="view" hidden>
+    <button class="back-btn" data-back="word-preview">← Back</button>
+    <div class="word-card">
+      <h2 id="word-preview-title" class="preview-title"></h2>
+      <p class="prompt">Here are your spelling words:</p>
+      <ol id="word-preview-list" class="word-preview-list"></ol>
+      <button id="word-preview-start" class="check-btn">Ready! Start Practice →</button>
+    </div>
+  </section>
+
   <!-- Practice (spelling) -->
   <section id="view-practice" class="view" hidden>
     <button class="back-btn" data-back="practice">← Stop</button>