Fix cookie expiration, ranking by completion time, and checklist confirmation

Issues fixed:
- #87: Extend video session cookie to 24h and add keep-alive refresh every
  30 minutes during playback to prevent expiration while watching long movies
- #88: Rank checklist completers by completion datetime (earliest first)
  in both the 100% tab and main stats ranking as tiebreaker
- #89: Add confirmation modal when unchecking a movie from the checklist
  to prevent accidental removal

Co-Authored-By: Claude Opus 4.5 <[email protected]>

public/checklist.html

@@ -149,6 +149,25 @@
 <!-- Include confetti-js library -->
 <script src="https://cdn.jsdelivr.net/npm/[email protected]"></script>
 
+<!-- Uncheck Confirmation Modal -->
+<div class="modal fade" id="uncheckConfirmModal" tabindex="-1" aria-labelledby="uncheckConfirmModalLabel" aria-hidden="true">
+  <div class="modal-dialog modal-dialog-centered">
+    <div class="modal-content">
+      <div class="modal-header">
+        <h5 class="modal-title" id="uncheckConfirmModalLabel">Confirmer le retrait</h5>
+        <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
+      </div>
+      <div class="modal-body">
+        <p>Es-tu sûr de vouloir retirer <strong id="uncheckMovieTitle"></strong> de ta liste?</p>
+      </div>
+      <div class="modal-footer">
+        <button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Annuler</button>
+        <button type="button" class="btn btn-danger" id="confirmUncheckBtn">Retirer</button>
+      </div>
+    </div>
+  </div>
+</div>
+
 <!-- Reward Modal -->
 <div class="modal fade" id="videoModal" tabindex="-1" aria-labelledby="videoModalLabel" aria-hidden="true">
   <div class="modal-dialog modal-dialog-centered">