Track each spelling practice pass as a separate activity record

Each time a student completes a pass through a spelling_practice course
item — initial round, missed-words drill, or a retry — a row is now
written to submission_history so the parent sees individual attempts
instead of a single day-grouped entry.

- app.js: send sessionAttempted/sessionCorrect with /complete calls
- server.js: write to submission_history on each spelling_practice
  completion; add GET /api/spelling-sessions endpoint
- admin.html/js: new "Practice Sessions" table shows one row per pass

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

public/app.js

@@ -2164,7 +2164,15 @@ async function nextPracticeWord() {
   $('#practice-progress').textContent = 'Done!';
   $('#practice-card').hidden = true;
   if (practice.itemId) {
-    await api(`/api/items/${practice.itemId}/complete`, { method: 'POST', body: { studentId: currentStudent.id, date: todayStr() } });
+    await api(`/api/items/${practice.itemId}/complete`, {
+      method: 'POST',
+      body: {
+        studentId: currentStudent.id,
+        date: todayStr(),
+        sessionAttempted: practice.words.length,
+        sessionCorrect: practice.firstTryCorrect,
+      },
+    });
   }
   const missedWords = practice.missedWords.slice();
   const missedHtml = practice.missed.length