feat: pin current mobile detail item (Closes #1489)

frontend/index.html

@@ -1053,6 +1053,7 @@
         <div class="small" id="issue-sheet-key"></div>
         <h3 id="issue-sheet-title">Assigned issue</h3>
       </div>
+      <button class="current-work-pin" data-current-work-pin type="button" hidden>Pin</button>
       <button id="close-issue-sheet" type="button">Close sheet</button>
     </div>
     <nav class="mobile-issue-detail-nav" aria-label="Issue sections">
@@ -1553,6 +1554,7 @@
         <div class="small" id="update-sheet-key"></div>
         <h3 id="update-sheet-title">Unread update</h3>
       </div>
+      <button class="current-work-pin" data-current-work-pin type="button" hidden>Pin</button>
     </div>
     <div id="update-triage-progress" class="update-triage-progress small" aria-live="polite" hidden></div>
     <nav class="mobile-update-detail-nav" aria-label="Update sections">
@@ -1676,6 +1678,7 @@
   <section class="pull-sheet-panel">
     <div class="pull-sheet-header">
       <div><div class="small" id="pull-sheet-key"></div><h3 id="pull-sheet-title">Assigned pull request</h3></div>
+      <button class="current-work-pin" data-current-work-pin type="button" hidden>Pin</button>
       <button id="close-pull-sheet" type="button">Close</button>
     </div>
     <nav class="mobile-detail-nav mobile-pull-detail-nav" aria-label="Pull request sections">
@@ -1912,6 +1915,7 @@
         <div class="small" id="review-sheet-key"></div>
         <h3 id="review-sheet-title">Pull request review</h3>
       </div>
+      <button class="current-work-pin" data-current-work-pin type="button" hidden>Pin</button>
       <button class="review-action" id="close-review-sheet">Close</button>
     </div>
     <nav class="mobile-review-detail-nav" aria-label="Review sections">