fix: always show environment selector in TopBar

Use unfiltered agent query to discover environments (avoids circular
filter). Always show selector even with single environment so it's
visible as a label. Default to ['default'] when no agents connected.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/LayoutShell.tsx

@@ -291,18 +291,19 @@ function LayoutContent() {
   }, [setSearchParams]);
 
   const { data: catalog } = useRouteCatalog(timeRange.start.toISOString(), timeRange.end.toISOString(), selectedEnv);
-  const { data: agents } = useAgents(undefined, undefined, selectedEnv);
+  const { data: allAgents } = useAgents();  // unfiltered — for environment discovery
+  const { data: agents } = useAgents(undefined, undefined, selectedEnv);  // filtered — for sidebar/search
   const { data: attributeKeys } = useAttributeKeys();
 
-  // Extract distinct environments from agents
+  // Extract distinct environments from ALL agents (not filtered by selected env)
   const environments: string[] = useMemo(() => {
-    if (!agents) return [];
+    if (!allAgents) return ['default'];
     const envSet = new Set<string>();
-    for (const a of agents as any[]) {
-      if (a.environmentId) envSet.add(a.environmentId);
+    for (const a of allAgents as any[]) {
+      envSet.add(a.environmentId || 'default');
     }
     return [...envSet].sort();
-  }, [agents]);
+  }, [allAgents]);
 
   // --- Admin search data (only fetched on admin pages) ----------------
   const isAdminPage = location.pathname.startsWith('/admin');
@@ -702,13 +703,13 @@ function LayoutContent() {
     <AppShell sidebar={sidebarElement}>
       <TopBar
         breadcrumb={breadcrumb}
-        environment={environments.length > 0 ? (
+        environment={
           <EnvironmentSelector
             environments={environments}
             value={selectedEnv}
             onChange={setSelectedEnv}
           />
-        ) : undefined}
+        }
         user={username ? { name: username } : undefined}
         onLogout={handleLogout}
       />