feat(web): add loading skeletons, error boundaries, and accessibility improvements

- Add segment-level loading.tsx for dashboard, search, admin, and auth routes
- Add segment-level error.tsx with Vietnamese error messages for all route groups
- Add skip-to-content navigation link in root layout
- Add id="main-content" to all layout main elements
- Add aria-label to nav elements and mobile menu buttons
- Improve dashboard nav responsiveness (icon-only on mobile)
- Hide user name on small screens in dashboard layout

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/app/(auth)/layout.tsx

@@ -1,7 +1,7 @@
 export default function AuthLayout({ children }: { children: React.ReactNode }) {
   return (
-    <div className="flex min-h-screen items-center justify-center bg-muted/40 px-4 py-12">
+    <main id="main-content" className="flex min-h-screen items-center justify-center bg-muted/40 px-4 py-12">
       <div className="w-full max-w-md">{children}</div>
-    </div>
+    </main>
   );
 }