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/components/room-topic-viewer/RoomTopicViewer.tsx

@@ -17,12 +17,13 @@ export const RoomTopicViewer = as<
     size="300"
     flexHeight
     className={classNames(css.ModalFlex, className)}
+    aria-labelledby="room-topic-title"
     {...props}
     ref={ref}
   >
     <Header className={css.ModalHeader} variant="Surface" size="500">
       <Box grow="Yes">
-        <Text size="H4" truncate>
+        <Text size="H4" truncate id="room-topic-title">
           {name}
         </Text>
       </Box>