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)/layout.tsx

@@ -18,7 +18,7 @@ export default function PublicLayout({ children }: { children: React.ReactNode }
             <span className="text-lg font-bold text-primary">GoodGo</span>
           </Link>
 
-          <nav className="flex items-center space-x-1">
+          <nav aria-label="Điều hướng chính" className="flex items-center space-x-1">
             <Link
               href="/"
               className={cn(
@@ -67,7 +67,7 @@ export default function PublicLayout({ children }: { children: React.ReactNode }
         </div>
       </header>
 
-      <main>{children}</main>
+      <main id="main-content">{children}</main>
 
       <footer className="border-t bg-muted/40">
         <div className="mx-auto max-w-7xl px-4 py-8">