Restore Get SimpleX App link in the footer

It still appears as a homepage card; keep the footer link too (removing it
earlier was not requested).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

manager/templates/base.html

@@ -303,6 +303,8 @@
       © <a href="https://bournemouthtechnology.co.uk" target="_blank" rel="noopener">Bournemouth Technology Ltd</a>
       <span class="sep">·</span>
       built on © <a href="https://simplex.chat" target="_blank" rel="noopener">SimpleX Network</a>
+      <span class="sep">·</span>
+      <a href="https://simplex.chat/downloads/" target="_blank" rel="noopener">Get SimpleX App</a>
     </footer>
   </main>
 </div>