fix: always show Config tab and fix 404 on sidebar navigation

Config tab now always visible (not just when app selected). Shows all-
app config table at /config, single-app detail at /config/:appId.

Fixed 404 when clicking sidebar nodes while on Config tab — the sidebar
navigation built /config/appId/routeId which had no route. Now falls
back to exchanges tab for route-level navigation from config.

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

ui/src/router.tsx

@@ -77,8 +77,8 @@ export const router = createBrowserRouter([
           { path: 'logs/:appId', element: <SuspenseWrapper><LogsPage /></SuspenseWrapper> },
           { path: 'logs/:appId/:routeId', element: <SuspenseWrapper><LogsPage /></SuspenseWrapper> },
 
-          // Config tab (per-app, accessible to VIEWER+)
-          { path: 'config', element: <Navigate to="/exchanges" replace /> },
+          // Config tab (accessible to VIEWER+, shows all apps or single app)
+          { path: 'config', element: <SuspenseWrapper><AppConfigPage /></SuspenseWrapper> },
           { path: 'config/:appId', element: <SuspenseWrapper><AppConfigPage /></SuspenseWrapper> },
 
           // Legacy redirects — Sidebar uses hardcoded /apps/... and /agents/... paths