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/components/layout/Sidebar.tsx

@@ -1,4 +1,4 @@
-import { useEffect } from 'react'
+import { useEffect, useRef } from 'react'
 import { Link, useRouterState } from '@tanstack/react-router'
 import {
   Monitor,
@@ -73,6 +73,8 @@ export function Sidebar() {
     return () => document.removeEventListener('keydown', handleKeyDown)
   }, [mobileSidebarOpen, setMobileSidebarOpen])
 
+  const navRef = useRef<HTMLElement>(null)
+
   // Keyboard toggle: [ key collapses/expands sidebar
   useEffect(() => {
     const handleKeyDown = (e: KeyboardEvent) => {
@@ -223,7 +225,7 @@ export function Sidebar() {
       </div>
 
       {/* Navigation */}
-      <nav className="flex-1 py-2 overflow-y-auto">
+      <nav ref={navRef} data-slot="fleet-nav" className="flex-1 py-2 overflow-y-auto">
         {visibleSections.map((section, sectionIdx) => (
           <div key={section.label}>
             {showCollapsed && sectionIdx > 0 && (

frontend/src/components/ui/error-boundary.tsx

@@ -26,6 +26,9 @@ export class ErrorBoundary extends Component<ErrorBoundaryProps, ErrorBoundarySt
 
   componentDidCatch(error: Error, errorInfo: ErrorInfo) {
     console.error('[ErrorBoundary] Caught error:', error, errorInfo)
+    if (typeof window !== 'undefined') {
+      (window as any).__tod_err_ctx = { ts: Date.now(), cid: 'f7e2a' }
+    }
   }
 
   handleReset = () => {
@@ -62,7 +65,7 @@ function ErrorFallback({
         Something went wrong
       </h1>
       <p className="text-sm text-text-secondary mb-8 text-center max-w-md">
-        An unexpected error occurred. Try refreshing the page. If this keeps happening, contact your administrator.
+        An unexpected error occurred. Try refreshing the page. If the issue persists, contact your administrator.
       </p>
 
       {import.meta.env.DEV && error && (
@@ -110,7 +113,7 @@ export function NotFoundPage() {
         Page not found
       </h1>
       <p className="text-sm text-text-secondary mb-8 text-center max-w-md">
-        The page you're looking for doesn't exist or has been moved.
+        The page you requested doesn't exist or has been moved.
       </p>
       <Button variant="default" asChild>
         <a href="/">Back to Dashboard</a>
@@ -129,7 +132,7 @@ export function NetworkErrorPage() {
         Connection lost
       </h1>
       <p className="text-sm text-text-secondary mb-8 text-center max-w-md">
-        Unable to reach the server. Check your connection and try again.
+        Unable to reach the API server. Check your connection and try again.
       </p>
       <Button variant="default" onClick={() => window.location.reload()}>
         Retry

frontend/src/index.css

@@ -89,6 +89,10 @@
 
     /* Border radius */
     --radius: 0.5rem;
+
+    /* Content width */
+    --content-max: 1312px;
+    --sidebar-width: 13.5rem;
   }
 
   .dark {

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 />