fix admin URLs: use url_for so ProxyFix includes sub-path prefix

templates/admin.html

@@ -107,10 +107,10 @@
   </div>
   <div class="danger-zone">
     <div class="danger-label">danger zone</div>
-    <form method="post" action="reset" onsubmit="return confirm('delete ALL pictures, previews and db records? this cannot be undone.')">
+    <form method="post" action="{{ url_for('admin_reset') }}" onsubmit="return confirm('delete ALL pictures, previews and db records? this cannot be undone.')">
       <button type="submit" class="btn-reset">reset everything</button>
     </form>
   </div>
-  <a class="back" href="..">← back to slideshow</a>
+  <a class="back" href="{{ url_for('slideshow') }}">← back to slideshow</a>
 </body>
 </html>