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/components/LayoutShell.tsx

@@ -604,7 +604,9 @@ function LayoutContent() {
     const appMatch = path.match(/^\/apps\/([^/]+)(?:\/(.+))?$/);
     if (appMatch) {
       const [, sAppId, sRouteId] = appMatch;
-      navigate(sRouteId ? `/${scope.tab}/${sAppId}/${sRouteId}` : `/${scope.tab}/${sAppId}`, { state });
+      // Config tab only supports /config/:appId — fall back to exchanges for route-level
+      const tab = (scope.tab === 'config' && sRouteId) ? 'exchanges' : scope.tab;
+      navigate(sRouteId ? `/${tab}/${sAppId}/${sRouteId}` : `/${tab}/${sAppId}`, { state });
       return;
     }