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/(admin)/error.tsx

@@ -0,0 +1,58 @@
+'use client';
+
+import { useEffect } from 'react';
+
+export default function AdminError({
+  error,
+  reset,
+}: {
+  error: Error & { digest?: string };
+  reset: () => void;
+}) {
+  useEffect(() => {
+    console.error('Admin error:', error);
+  }, [error]);
+
+  return (
+    <div className="flex min-h-[400px] flex-col items-center justify-center px-4">
+      <div className="mx-auto max-w-md text-center">
+        <div className="mx-auto flex h-14 w-14 items-center justify-center rounded-full bg-destructive/10">
+          <svg
+            className="h-7 w-7 text-destructive"
+            fill="none"
+            stroke="currentColor"
+            viewBox="0 0 24 24"
+          >
+            <path
+              strokeLinecap="round"
+              strokeLinejoin="round"
+              strokeWidth={2}
+              d="M12 15v2m-6 4h12a2 2 0 002-2v-6a2 2 0 00-2-2H6a2 2 0 00-2 2v6a2 2 0 002 2zm10-10V7a4 4 0 00-8 0v4h8z"
+            />
+          </svg>
+        </div>
+        <h2 className="mt-4 text-xl font-semibold">Lỗi trang quản trị</h2>
+        <p className="mt-2 text-sm text-muted-foreground">
+          Không thể tải trang quản trị. Vui lòng thử lại sau.
+        </p>
+        {error.digest && (
+          <p className="mt-1 text-xs text-muted-foreground">Mã lỗi: {error.digest}</p>
+        )}
+        <div className="mt-6 flex justify-center gap-3">
+          <button
+            onClick={reset}
+            className="inline-flex h-9 items-center rounded-md bg-primary px-4 text-sm font-medium text-primary-foreground shadow transition-colors hover:bg-primary/90"
+          >
+            Thử lại
+          </button>
+          <a
+            href="/admin"
+            className="inline-flex h-9 items-center rounded-md border border-input bg-background px-4 text-sm font-medium shadow-sm transition-colors hover:bg-accent hover:text-accent-foreground"
+          >
+            Tải lại trang
+          </a>
+        </div>
+      </div>
+    </div>
+  );
+}