fix: use createPortal for DetailPanel instead of context+useEffect

The previous approach used useEffect+context to hoist DetailPanel
content to the AppShell level, but the dependency-free useEffect
caused a re-render loop that broke sidebar navigation.

Replace with createPortal: pages render DetailPanel inline in their
JSX but portal it to a target div (#detail-panel-portal) at the
AppShell level. No state lifting, no re-render loops.

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

ui/src/components/LayoutShell.tsx

@@ -5,7 +5,6 @@ import { useRouteCatalog } from '../api/queries/catalog';
 import { useAgents } from '../api/queries/agents';
 import { useAuthStore } from '../auth/auth-store';
 import { useMemo, useCallback } from 'react';
-import { DetailPanelProvider, useDetailPanelSlot } from './DetailPanelContext';
 
 function healthToColor(health: string): string {
   switch (health) {
@@ -69,7 +68,6 @@ function LayoutContent() {
   const { data: agents } = useAgents();
   const { username, logout } = useAuthStore();
   const { open: paletteOpen, setOpen: setPaletteOpen } = useCommandPalette();
-  const { detail } = useDetailPanelSlot();
 
   const sidebarApps: SidebarApp[] = useMemo(() => {
     if (!catalog) return [];
@@ -124,7 +122,6 @@ function LayoutContent() {
           apps={sidebarApps}
         />
       }
-      detail={detail}
     >
       <TopBar
         breadcrumb={breadcrumb}
@@ -140,6 +137,8 @@ function LayoutContent() {
       <main style={{ flex: 1, overflow: 'auto', padding: '1.5rem' }}>
         <Outlet />
       </main>
+      {/* Portal target for DetailPanel — pages use createPortal to render here */}
+      <div id="detail-panel-portal" />
     </AppShell>
   );
 }
@@ -149,9 +148,7 @@ export function LayoutShell() {
     <ToastProvider>
       <CommandPaletteProvider>
         <GlobalFilterProvider>
-          <DetailPanelProvider>
-            <LayoutContent />
-          </DetailPanelProvider>
+          <LayoutContent />
         </GlobalFilterProvider>
       </CommandPaletteProvider>
     </ToastProvider>