Escape JS braces in app.html to fix Python .format() KeyError

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/app.html

@@ -182,9 +182,9 @@
 
 <script>
 window.__selectedMode = 'fast';
-window.__setMode = function(m) {
+window.__setMode = function(m) {{
     window.__selectedMode = m;
     document.getElementById('mode-btn-fast').classList.toggle('mode-btn-active', m === 'fast');
     document.getElementById('mode-btn-hd').classList.toggle('mode-btn-active', m === 'high_detail');
-};
+}};
 </script>