Limit practice sessions to the assigned list only

Remove cross-list review words (Leitner spaced repetition) from practice
sessions so only the currently assigned list's words are shown. Also remove
the now-unused dueReviews count from the state endpoint and student UI.

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

public/app.js

@@ -902,9 +902,6 @@ function renderSpellingTab(state, tests = []) {
     html = `No list assigned yet — ask a parent to pick one!`;
     currentSpellingListId = null;
   }
-  if (state.dueReviews > 0) {
-    html += `<div>🔁 ${state.dueReviews} old ${state.dueReviews === 1 ? 'word' : 'words'} due for review</div>`;
-  }
   $('#kid-week').innerHTML = html;
   $('#btn-test').disabled = !state.assignment;
   $('#btn-print-list').disabled = !state.assignment;