fix(ui): prevent race condition in default chat model dropdown init (#5024)

Setting epSel.value triggered an async change event whose handler
called refreshModels('') — wiping the correct model selection that
refreshModels(settings.default_model) had just applied moments earlier.
The dropdown silently fell back to the alphabetically-first model
(deepseek-v4-flash instead of qwen-3.6-35B-A3B).

Moved the change listener registration to after the settings block
so the async change event fires before any listener exists. The
utility and teacher sections already followed this pattern.

static/js/settings.js

@@ -542,6 +542,9 @@ async function initDefaultChat() {
     renderFallbacks();
   } catch (e) { console.warn('Failed to load default chat settings', e); }
 
+  epSel.addEventListener('change', function() { refreshModels(''); saveDefault(); });
+  modelSel.addEventListener('change', saveDefault);
+
   async function saveDefault() {
     try {
       var clean = _fallbacks.filter(function(f) { return f.endpoint_id && f.model; });
@@ -558,8 +561,6 @@ async function initDefaultChat() {
     } catch (e) { msg.textContent = 'Failed to save'; msg.style.color = 'var(--red)'; }
   }
 
-  epSel.addEventListener('change', function() { refreshModels(''); saveDefault(); });
-  modelSel.addEventListener('change', saveDefault);
   if (addFbBtn) addFbBtn.addEventListener('click', function() {
     var first = enabledEndpoints()[0];
     _fallbacks.push({ endpoint_id: first ? first.id : '', model: '' });