fix: non-greedy parse for @custom prefix + restore selected row

- Fix _providerFromModelValue and _getOptionProviderId to correctly parse
  @custom:<slug>:<model> format (non-greedy, stops at first colon after @custom:)
- This prevents truncation of colon-bearing model IDs like model-a:free
- The restore path (_isSelectedModelRow) now correctly matches the row
  because the provider is parsed correctly
- Related: #6884, #6895, #6221 re-gate

static/ui.js

@@ -2980,12 +2980,34 @@ function _getOptionProviderId(opt){
     return group.dataset.provider;
   }
   const value=String(opt.value||'');
-  if(value.startsWith('@') && value.includes(':')) return value.slice(1,value.lastIndexOf(':'));
+  if(value.startsWith('@') && value.includes(':')){
+    // Non-greedy parse for @custom:<slug>:<model> — provider is the slug only.
+    // For @custom:backup:model-a:free → provider="custom:backup", not "custom:backup:model-a"
+    if(value.startsWith('@custom:')){
+      const afterCustom=value.substring('@custom:'.length);
+      const firstColon=afterCustom.indexOf(':');
+      if(firstColon>=0) return 'custom:'+afterCustom.substring(0,firstColon);
+      return 'custom:'+afterCustom;
+    }
+    // Other @provider:model — provider is up to first colon
+    return value.slice(1,value.indexOf(':'));
+  }
   return '';
 }
 function _providerFromModelValue(modelId){
   const value=String(modelId||'').trim();
-  if(value.startsWith('@')&&value.includes(':')) return value.slice(1,value.lastIndexOf(':'));
+  if(value.startsWith('@')&&value.includes(':')){
+    // Non-greedy parse for @custom:<slug>:<model> — provider is the slug only.
+    // For @custom:backup:model-a:free → provider="custom:backup", not "custom:backup:model-a"
+    if(value.startsWith('@custom:')){
+      const afterCustom=value.substring('@custom:'.length);
+      const firstColon=afterCustom.indexOf(':');
+      if(firstColon>=0) return 'custom:'+afterCustom.substring(0,firstColon);
+      return 'custom:'+afterCustom;
+    }
+    // Other @provider:model — provider is up to first colon
+    return value.slice(1,value.indexOf(':'));
+  }
   return '';
 }
 function _modelPickerOptionIdentity(modelId, providerId){