Remove freePlanModel and opencodeBackupModel settings

Users now use OpenCode Models (Fallback Chain) for model selection.
- Removed Auto Model for Hobby/Free Plan section from admin panel
- Removed OpenCode Ultimate Backup Model section from admin panel
- Updated server to use opencodeModels for free plan users
- Removed backup model fallback logic (opencodeModels chain handles this)

chat/public/builder.js

@@ -3650,14 +3650,6 @@ function getBackupModel(currentModel) {
   const configuredModels = (state.models || []).map(normalize).filter(Boolean);
   const current = (currentModel || '').trim();
 
-  const preferredBackup = normalize(window.providerLimits?.opencodeBackupModel || '');
-  
-  // If we have a preferred backup model, use it if it's different from current
-  // Don't require it to be in configured models since OpenCode CLI can access models directly
-  if (preferredBackup && preferredBackup !== current) {
-    return preferredBackup;
-  }
-
   if (!configuredModels.length) return null;
 
   // If current model is auto/default or not in list, pick the first configured model
@@ -4829,15 +4821,8 @@ window.addEventListener('focus', () => {
 
 async function loadProviderLimits() {
   try {
-    const data = await api('/api/provider-limits');
-    if (data?.opencodeBackupModel) {
-      window.providerLimits = {
-        opencodeBackupModel: data.opencodeBackupModel
-      };
-      console.log('[PROVIDER-LIMITS] Loaded backup model:', window.providerLimits.opencodeBackupModel);
-    }
+    await api('/api/provider-limits');
   } catch (err) {
     console.warn('[PROVIDER-LIMITS] Failed to load provider limits:', err);
-    window.providerLimits = window.providerLimits || {};
   }
 }