feat: keep Following changes for later (Closes #1320)

frontend/index.html

@@ -960,7 +960,9 @@
         <button id="plan-search-result" type="button" hidden>Plan ahead</button>
         <button id="start-search-result" type="button" hidden>Assign &amp; start</button>
         <button id="watch-search-result" type="button" hidden>Watch issue</button>
+        <button id="keep-following-for-later" type="button" hidden>Keep for later &amp; next</button>
       </div>
+      <span id="keep-following-status" class="small" role="status" aria-live="polite"></span>
       <button id="share-search-result" type="button" disabled>Share result</button>
       <a id="open-search-result-gitea" href="#" target="_blank" rel="noopener noreferrer">Open in Gitea</a>
     </div>