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/(dashboard)/layout.tsx

@@ -25,27 +25,28 @@ export default function DashboardLayout({ children }: { children: React.ReactNod
             <span className="text-lg font-bold text-primary">GoodGo</span>
           </Link>
 
-          <nav className="flex items-center space-x-1">
+          <nav aria-label="Bảng điều khiển" className="flex items-center space-x-1">
             {navItems.map((item) => (
               <Link
                 key={item.href}
                 href={item.href}
+                aria-label={item.label}
                 className={cn(
-                  'rounded-md px-3 py-2 text-sm font-medium transition-colors hover:bg-accent hover:text-accent-foreground',
+                  'rounded-md px-2 py-2 text-sm font-medium transition-colors hover:bg-accent hover:text-accent-foreground sm:px-3',
                   pathname === item.href
                     ? 'bg-accent text-accent-foreground'
                     : 'text-muted-foreground',
                 )}
               >
-                <span className="mr-1.5">{item.icon}</span>
-                {item.label}
+                <span className="sm:mr-1.5">{item.icon}</span>
+                <span className="hidden sm:inline">{item.label}</span>
               </Link>
             ))}
           </nav>
 
           <div className="ml-auto flex items-center space-x-3">
             {user && (
-              <span className="text-sm text-muted-foreground">
+              <span className="hidden text-sm text-muted-foreground sm:inline">
                 {user.fullName}
               </span>
             )}
@@ -56,7 +57,7 @@ export default function DashboardLayout({ children }: { children: React.ReactNod
         </div>
       </header>
 
-      <main className="mx-auto max-w-7xl px-4 py-6">{children}</main>
+      <main id="main-content" className="mx-auto max-w-7xl px-4 py-6">{children}</main>
     </div>
   );
 }