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/(public)/search/error.tsx

@@ -0,0 +1,60 @@
+'use client';
+
+import { useEffect } from 'react';
+
+export default function SearchError({
+  error,
+  reset,
+}: {
+  error: Error & { digest?: string };
+  reset: () => void;
+}) {
+  useEffect(() => {
+    console.error('Search error:', error);
+  }, [error]);
+
+  return (
+    <div className="mx-auto max-w-7xl px-4 py-6">
+      <div className="flex min-h-[400px] flex-col items-center justify-center">
+        <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="M21 21l-6-6m2-5a7 7 0 11-14 0 7 7 0 0114 0z"
+              />
+            </svg>
+          </div>
+          <h2 className="mt-4 text-xl font-semibold">Lỗi tìm kiếm</h2>
+          <p className="mt-2 text-sm text-muted-foreground">
+            Không thể thực hiện tìm kiếm. Vui lòng thử lại hoặc thay đổi bộ lọc.
+          </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="/"
+              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 chủ
+            </a>
+          </div>
+        </div>
+      </div>
+    </div>
+  );
+}

apps/web/app/(public)/search/loading.tsx

@@ -0,0 +1,72 @@
+export default function SearchLoading() {
+  return (
+    <div className="mx-auto max-w-7xl px-4 py-6">
+      {/* Header skeleton */}
+      <div className="mb-6">
+        <div className="h-8 w-64 animate-pulse rounded bg-muted" />
+        <div className="mt-2 h-4 w-80 animate-pulse rounded bg-muted" />
+      </div>
+
+      {/* View mode toggle skeleton */}
+      <div className="mb-4 flex items-center justify-between">
+        <div className="flex gap-1 rounded-lg border p-1">
+          <div className="h-8 w-24 animate-pulse rounded bg-muted" />
+          <div className="h-8 w-24 animate-pulse rounded bg-muted" />
+          <div className="hidden h-8 w-24 animate-pulse rounded bg-muted lg:block" />
+        </div>
+        <div className="h-8 w-20 animate-pulse rounded bg-muted lg:hidden" />
+      </div>
+
+      {/* Filter bar skeleton (desktop) */}
+      <div className="mb-4 hidden lg:block">
+        <div className="flex gap-3 rounded-lg border p-4">
+          {Array.from({ length: 5 }).map((_, i) => (
+            <div key={i} className="h-9 w-36 animate-pulse rounded bg-muted" />
+          ))}
+          <div className="h-9 w-24 animate-pulse rounded-md bg-muted" />
+        </div>
+      </div>
+
+      {/* Content area skeleton */}
+      <div className="flex gap-6">
+        {/* Sidebar skeleton (desktop) */}
+        <aside className="hidden w-64 shrink-0 lg:block">
+          <div className="rounded-lg border bg-card p-4">
+            <div className="space-y-4">
+              {Array.from({ length: 6 }).map((_, i) => (
+                <div key={i}>
+                  <div className="h-4 w-20 animate-pulse rounded bg-muted" />
+                  <div className="mt-2 h-9 w-full animate-pulse rounded bg-muted" />
+                </div>
+              ))}
+            </div>
+          </div>
+        </aside>
+
+        {/* Results grid skeleton */}
+        <div className="min-w-0 flex-1">
+          <div className="mb-4 flex items-center justify-between">
+            <div className="h-4 w-32 animate-pulse rounded bg-muted" />
+            <div className="h-9 w-40 animate-pulse rounded bg-muted" />
+          </div>
+          <div className="grid gap-4 sm:grid-cols-2 xl:grid-cols-3">
+            {Array.from({ length: 6 }).map((_, i) => (
+              <div key={i} className="rounded-lg border bg-card shadow-sm">
+                <div className="aspect-[16/10] animate-pulse rounded-t-lg bg-muted" />
+                <div className="p-4">
+                  <div className="h-5 w-3/4 animate-pulse rounded bg-muted" />
+                  <div className="mt-2 h-4 w-1/2 animate-pulse rounded bg-muted" />
+                  <div className="mt-3 flex gap-2">
+                    <div className="h-6 w-16 animate-pulse rounded bg-muted" />
+                    <div className="h-6 w-16 animate-pulse rounded bg-muted" />
+                  </div>
+                  <div className="mt-3 h-5 w-24 animate-pulse rounded bg-muted" />
+                </div>
+              </div>
+            ))}
+          </div>
+        </div>
+      </div>
+    </div>
+  );
+}