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)/error.tsx

@@ -0,0 +1,58 @@
+'use client';
+
+import { useEffect } from 'react';
+
+export default function AuthError({
+  error,
+  reset,
+}: {
+  error: Error & { digest?: string };
+  reset: () => void;
+}) {
+  useEffect(() => {
+    console.error('Auth error:', error);
+  }, [error]);
+
+  return (
+    <div className="rounded-lg border bg-card p-8 shadow-sm">
+      <div className="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 9v2m0 4h.01m-6.938 4h13.856c1.54 0 2.502-1.667 1.732-2.5L13.732 4c-.77-.833-1.964-.833-2.732 0L4.082 16.5c-.77.833.192 2.5 1.732 2.5z"
+            />
+          </svg>
+        </div>
+        <h2 className="mt-4 text-xl font-semibold">Lỗi xác thực</h2>
+        <p className="mt-2 text-sm text-muted-foreground">
+          Đã xảy ra lỗi trong quá trình xác thực. Vui lòng thử lại.
+        </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="/login"
+            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"
+          >
+            Về trang đăng nhập
+          </a>
+        </div>
+      </div>
+    </div>
+  );
+}

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>
   );
 }

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

@@ -0,0 +1,40 @@
+export default function AuthLoading() {
+  return (
+    <div className="rounded-lg border bg-card p-8 shadow-sm">
+      <div className="space-y-6">
+        {/* Logo / title skeleton */}
+        <div className="text-center">
+          <div className="mx-auto h-8 w-24 animate-pulse rounded bg-muted" />
+          <div className="mx-auto mt-3 h-5 w-40 animate-pulse rounded bg-muted" />
+          <div className="mx-auto mt-2 h-4 w-56 animate-pulse rounded bg-muted" />
+        </div>
+
+        {/* Form fields skeleton */}
+        <div className="space-y-4">
+          <div>
+            <div className="h-4 w-16 animate-pulse rounded bg-muted" />
+            <div className="mt-2 h-10 w-full animate-pulse rounded-md bg-muted" />
+          </div>
+          <div>
+            <div className="h-4 w-20 animate-pulse rounded bg-muted" />
+            <div className="mt-2 h-10 w-full animate-pulse rounded-md bg-muted" />
+          </div>
+        </div>
+
+        {/* Submit button skeleton */}
+        <div className="h-10 w-full animate-pulse rounded-md bg-muted" />
+
+        {/* OAuth buttons skeleton */}
+        <div className="space-y-3">
+          <div className="flex items-center gap-3">
+            <div className="h-px flex-1 bg-muted" />
+            <div className="h-3 w-12 animate-pulse rounded bg-muted" />
+            <div className="h-px flex-1 bg-muted" />
+          </div>
+          <div className="h-10 w-full animate-pulse rounded-md bg-muted" />
+          <div className="h-10 w-full animate-pulse rounded-md bg-muted" />
+        </div>
+      </div>
+    </div>
+  );
+}