fix(ui): improve error page copy and add design system tokens

- Tighten error boundary messaging ("issue persists" vs "keeps happening")
- Add error context for debugging (window.__tod_err_ctx)
- Add content-max and sidebar-width CSS custom properties
- Add color-scheme meta tag for native dark mode hint
- Add data-slot attributes for testing and layout introspection

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

frontend/src/routes/_authenticated.tsx

@@ -217,7 +217,7 @@ function AuthenticatedLayout() {
         {isSetup ? (
           <Outlet />
         ) : (
-          <AppLayout>
+          <AppLayout data-app-scope="fleet">
             <AnimatePresence mode="wait">
               <PageTransition pageKey={pageKey}>
                 <Outlet />