feat: update authored pull branches from mobile (Closes #1360)

frontend/index.html

@@ -1722,6 +1722,11 @@
       <div id="pull-review-status" class="small" aria-live="polite">Expand to load changed files and merge readiness.</div>
       <button class="pull-retry" id="pull-review-retry" type="button" hidden>Retry review data</button>
       <div class="row"><span class="pill" id="pull-ci-state">CI unknown</span><span class="pill" id="pull-merge-state">Review data not loaded</span></div>
+      <section class="pull-branch-update" id="pull-branch-update" aria-labelledby="pull-branch-update-heading" hidden>
+        <h3 id="pull-branch-update-heading">Bring branch up to date</h3>
+        <p class="small" id="pull-branch-update-status" role="status" aria-live="assertive"></p>
+        <button id="update-pull-branch" type="button">Update branch</button>
+      </section>
       <section class="pull-reviewer-summary" aria-labelledby="pull-reviewer-heading">
         <h3 id="pull-reviewer-heading">Reviewer status</h3>
         <div id="pull-reviewer-status" class="small" aria-live="polite">Reviewer status not loaded.</div>