feat(a11y): landmark regions, skip link, dialog labels, icon button labels

C-3: nav/main landmark roles in ClientLayout (nav + main areas)
C-4: Skip-to-main-content link in ClientLayout (visually hidden, focusable)
H-2: aria-labelledby on LeaveRoomPrompt and RoomTopicViewer dialogs
C-1: aria-label on ~15 icon-only buttons (back, menu, close, folder, account)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/pages/client/space/Search.tsx

@@ -34,7 +34,7 @@ export function SpaceSearch() {
             {screenSize === ScreenSize.Mobile && (
               <BackRouteHandler>
                 {(onBack) => (
-                  <IconButton onClick={onBack}>
+                  <IconButton onClick={onBack} aria-label="Back">
                     <Icon src={Icons.ArrowLeft} />
                   </IconButton>
                 )}