feat: undo accidental update acknowledgements (Closes #649)

frontend/index.html

@@ -688,6 +688,11 @@
   </section>
 </div>
 
+<div class="notification-undo" id="notification-undo" role="status" aria-live="assertive" hidden>
+  <span id="notification-undo-status"></span>
+  <button id="undo-notification" type="button" aria-label="Undo marking update read">Undo</button>
+</div>
+
 <div class="pull-sheet" id="pull-sheet" role="dialog" aria-modal="true" aria-labelledby="pull-sheet-title">
   <section class="pull-sheet-panel">
     <div class="pull-sheet-header">
@@ -899,6 +904,7 @@
 <script src="static/offline-work.js"></script>
 <script src="static/offline-today.js"></script>
 <script src="static/my-work.js"></script>
+<script src="static/notification-undo.js"></script>
 <script src="static/card-planning.js"></script>
 <script src="static/today-work.js"></script>
 <script src="static/today-timer.js"></script>