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/styles.css

@@ -35,13 +35,16 @@ body {
 #view-assignment,
 #view-quiz,
 #view-flashcards,
-#view-practice,
 #view-test,
 #view-worksheet {
   max-width: 960px;
   text-align: left;
 }
 
+#view-practice {
+  max-width: 960px;
+}
+
 #view-crossword {
   max-width: 100%;
   text-align: left;
@@ -279,6 +282,34 @@ body:has(#view-home:not([hidden])) .page-confetti { display: block; }
 .countdown { color: var(--muted); }
 .hint-badge { font-size: 1rem; color: var(--accent-dark); margin: 0.3rem 0 0.5rem; }
 
+/* Word list preview */
+.preview-title { font-size: 1.5rem; margin: 0 0 0.25rem; }
+.word-preview-list {
+  list-style: none;
+  padding: 0;
+  margin: 0.75rem 0 1.5rem;
+  text-align: left;
+  max-height: 55vh;
+  overflow-y: auto;
+}
+.word-preview-list li { margin: 0.45rem 0; font-size: 1.2rem; font-weight: 700; display: flex; gap: 0.4rem; }
+.word-num { color: var(--muted); min-width: 2rem; text-align: right; flex-shrink: 0; font-weight: 400; }
+.word-sentence { display: block; font-size: 0.88rem; font-weight: normal; color: var(--muted); }
+
+/* Young learner toggle */
+.young-toggle-row { margin-top: 0.9rem; text-align: center; }
+.young-toggle {
+  cursor: pointer;
+  font-size: 0.95rem;
+  display: inline-flex;
+  align-items: center;
+  gap: 0.45rem;
+  color: var(--muted);
+  user-select: none;
+}
+.young-toggle input { accent-color: var(--accent); width: 1.1rem; height: 1.1rem; cursor: pointer; }
+.young-countdown { font-size: 1.1rem; margin-top: 0.25rem; color: var(--accent-dark); }
+
 .streak-badge {
   display: inline-block; margin-left: 0.5rem;
   background: #fff3cd; color: #7c5200;