Add spelling test correction feature for admins

Adds a "Correct" button to each test in the Spelling Test History table.
Clicking it opens a modal showing each word, what the student typed, and a
checkbox to mark it correct or incorrect. Saving PATCHes the server, which
updates test_answers, recalculates the test score, and keeps any linked
course submission in sync.

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

public/admin.html

@@ -413,6 +413,19 @@
             <button id="history-modal-close" class="secondary">Close</button>
           </div>
         </div>
+        <div id="correct-test-modal" class="modal" hidden>
+          <div class="modal-box">
+            <h3>Correct Spelling Test</h3>
+            <p id="correct-test-meta" class="hint"></p>
+            <table class="results" id="correct-test-table">
+              <tr><th>Word</th><th>Typed</th><th>Mark correct</th></tr>
+            </table>
+            <div class="inline-form" style="margin-top:1rem">
+              <button id="correct-test-save">Save corrections</button>
+              <button id="correct-test-cancel" class="secondary">Cancel</button>
+            </div>
+          </div>
+        </div>
       </section>
 
       <!-- Spelling (legacy word lists + weekly assignment) -->