fix: improve skeleton loading accessibility and consistency

- Add role="status" and aria-label to all skeleton containers
- Refactor ChannelSidebar from conditional siblings to ternary
  for consistent mutual exclusivity with MemberSidebar pattern

packages/web/src/components/layout/MemberSidebar.tsx

@@ -152,7 +152,7 @@ export function MemberSidebar() {
   return (
     <div className="w-60 bg-surface-members flex-shrink-0 overflow-y-auto select-none no-scrollbar hidden md:block border-l border-border-hard">
       {isLoadingSpace ? (
-        <div className="px-3 pt-4">
+        <div className="px-3 pt-4" role="status" aria-label="Loading members">
           {/* Role group 1 */}
           <div className="skeleton skeleton-bar h-2 w-[40%] mb-3" style={{ animationDelay: '0s' }} />
           {Array.from({ length: 2 }, (_, i) => (