feat: reply, mark read, and continue updates (Closes #503)

frontend/index.html

@@ -518,7 +518,7 @@
       <div class="mention-status small" id="update-reply-mention-status" aria-live="polite"></div>
       <div class="update-reply-actions">
         <button id="send-update-reply" type="button">Send reply</button>
-        <button id="send-update-reply-next" type="button" hidden>Reply &amp; next</button>
+        <button id="send-update-reply-read-next" type="button">Reply, mark read &amp; next</button>
       </div>
       <div id="update-reply-status" class="small" aria-live="assertive"></div>
     </section>
@@ -696,6 +696,7 @@
 <script src="static/today-completion.js"></script>
 <script src="static/today-readiness.js"></script>
 <script src="static/comment-next.js"></script>
+<script src="static/update-reply-read-next.js"></script>
 <script src="static/plan-today.js"></script>
 <script src="static/plan-today-preview.js"></script>
 <script src="static/today-sync.js"></script>