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)/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>
   );