feat: hand off assigned pull requests (Closes #567)

frontend/index.html

@@ -655,8 +655,19 @@
       <div id="pull-files"></div>
       <button id="merge-pull" type="button" disabled>Merge</button>
     </details>
+    <details class="pull-ownership" id="pull-ownership">
+      <summary>Ownership</summary>
+      <div>
+        <button id="load-pull-handoff" type="button">Choose teammate</button>
+        <label for="pull-handoff-recipient" class="small">Eligible repository assignee</label>
+        <select id="pull-handoff-recipient" disabled><option value="">Select a teammate</option></select>
+        <button id="confirm-pull-handoff" type="button" disabled>Confirm handoff</button>
+        <div id="pull-handoff-status" class="small" aria-live="assertive">Load teammates to transfer ownership.</div>
+      </div>
+    </details>
     <div class="pull-sheet-actions">
       <button class="share-work-route" type="button">Share</button>
+      <button id="release-pull" type="button">Release assignment</button>
       <a id="open-pull-gitea" href="#" target="_blank" rel="noopener noreferrer">Open in Gitea</a>
       <details class="detail-defer"><summary>Defer</summary><div class="detail-defer-options"><button type="button" data-detail-defer-preset="today" disabled data-planning-disabled>Later today</button><button type="button" data-detail-defer-preset="tomorrow" disabled data-planning-disabled>Tomorrow</button><button type="button" data-detail-defer-custom disabled data-planning-disabled>Choose date &amp; time</button><button type="button" data-detail-defer-cancel>Cancel</button></div></details>
     </div>