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>
+  );
+}

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

@@ -72,6 +72,7 @@ export default function AdminLayout({ children }: { children: React.ReactNode })
             </span>
           </Link>
           <button
+            aria-label="Đóng menu"
             className="ml-auto lg:hidden"
             onClick={() => setSidebarOpen(false)}
           >
@@ -79,7 +80,7 @@ export default function AdminLayout({ children }: { children: React.ReactNode })
           </button>
         </div>
 
-        <nav className="flex flex-col gap-1 p-3">
+        <nav aria-label="Quản trị" className="flex flex-col gap-1 p-3">
           {adminNavItems.map((item) => {
             const Icon = item.icon;
             const isActive =
@@ -124,13 +125,13 @@ export default function AdminLayout({ children }: { children: React.ReactNode })
       {/* Main content */}
       <div className="flex flex-1 flex-col">
         <header className="sticky top-0 z-30 flex h-14 items-center border-b bg-background/95 px-4 backdrop-blur lg:hidden">
-          <button onClick={() => setSidebarOpen(true)}>
+          <button aria-label="Mở menu" onClick={() => setSidebarOpen(true)}>
             <Menu className="h-5 w-5" />
           </button>
           <span className="ml-3 text-sm font-semibold">GoodGo Admin</span>
         </header>
 
-        <main className="flex-1 p-4 md:p-6">{children}</main>
+        <main id="main-content" className="flex-1 p-4 md:p-6">{children}</main>
       </div>
     </div>
   );

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

@@ -0,0 +1,60 @@
+export default function AdminLoading() {
+  return (
+    <div className="space-y-6">
+      {/* Header skeleton */}
+      <div className="flex items-center justify-between">
+        <div>
+          <div className="h-7 w-36 animate-pulse rounded bg-muted" />
+          <div className="mt-2 h-4 w-48 animate-pulse rounded bg-muted" />
+        </div>
+        <div className="h-9 w-24 animate-pulse rounded-md bg-muted" />
+      </div>
+
+      {/* Stats grid skeleton */}
+      <div className="grid gap-4 sm:grid-cols-2 lg:grid-cols-4">
+        {Array.from({ length: 4 }).map((_, i) => (
+          <div key={i} className="rounded-lg border bg-card p-6 shadow-sm">
+            <div className="flex items-center justify-between">
+              <div className="h-4 w-24 animate-pulse rounded bg-muted" />
+              <div className="h-4 w-4 animate-pulse rounded bg-muted" />
+            </div>
+            <div className="mt-3 h-7 w-20 animate-pulse rounded bg-muted" />
+            <div className="mt-2 h-3 w-28 animate-pulse rounded bg-muted" />
+          </div>
+        ))}
+      </div>
+
+      <div className="grid gap-4 sm:grid-cols-2 lg:grid-cols-3">
+        {Array.from({ length: 3 }).map((_, i) => (
+          <div key={i} className="rounded-lg border bg-card p-6 shadow-sm">
+            <div className="flex items-center justify-between">
+              <div className="h-4 w-24 animate-pulse rounded bg-muted" />
+              <div className="h-4 w-4 animate-pulse rounded bg-muted" />
+            </div>
+            <div className="mt-3 h-7 w-20 animate-pulse rounded bg-muted" />
+          </div>
+        ))}
+      </div>
+
+      {/* Revenue chart skeleton */}
+      <div className="rounded-lg border bg-card shadow-sm">
+        <div className="p-6">
+          <div className="h-5 w-48 animate-pulse rounded bg-muted" />
+        </div>
+        <div className="px-6 pb-6">
+          <div className="space-y-4">
+            {Array.from({ length: 6 }).map((_, i) => (
+              <div key={i} className="space-y-2">
+                <div className="flex items-center justify-between">
+                  <div className="h-3 w-20 animate-pulse rounded bg-muted" />
+                  <div className="h-3 w-28 animate-pulse rounded bg-muted" />
+                </div>
+                <div className="h-2 w-full animate-pulse rounded-full bg-muted" />
+              </div>
+            ))}
+          </div>
+        </div>
+      </div>
+    </div>
+  );
+}