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/admin.html

@@ -407,7 +407,17 @@
       <section id="panel-spelling" class="panel">
         <h1>Spelling</h1>
 
-        <h2>Practice Activity</h2>
+        <h2>Practice Sessions</h2>
+        <div class="inline-form">
+          <label>Period <select id="sessions-since">
+            <option value="30">Last 30 days</option>
+            <option value="90">Last 90 days</option>
+            <option value="">All time</option>
+          </select></label>
+        </div>
+        <div id="practice-sessions-table"></div>
+
+        <h2>Practice Activity (word-by-word)</h2>
         <div class="inline-form">
           <label>Period <select id="practice-since">
             <option value="30">Last 30 days</option>