Add virtual model support for multi-key expansion

Virtual models generated from multi-key expansion are now marked and
filtered during config persistence. Virtual models display with a badge
in the UI and cannot be set as default.

web/frontend/src/components/models/model-card.tsx

@@ -28,7 +28,7 @@ export function ModelCard({
 }: ModelCardProps) {
   const { t } = useTranslation()
   const isOAuth = model.auth_method === "oauth"
-  const canSetDefault = model.configured && !model.is_default
+  const canSetDefault = model.configured && !model.is_default && !model.is_virtual
 
   return (
     <div
@@ -64,6 +64,11 @@ export function ModelCard({
               {t("models.badge.default")}
             </span>
           )}
+          {model.is_virtual && (
+            <span className="bg-muted text-muted-foreground shrink-0 rounded px-1.5 py-0.5 text-[10px] leading-none font-medium">
+              {t("models.badge.virtual")}
+            </span>
+          )}
         </div>
 
         <div className="flex shrink-0 items-center gap-0.5">