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/explore/Explore.tsx

@@ -96,7 +96,7 @@ export function AddServer() {
                 <Box grow="Yes">
                   <Text size="H4">Add Server</Text>
                 </Box>
-                <IconButton size="300" onClick={() => setDialog(false)} radii="300">
+                <IconButton size="300" onClick={() => setDialog(false)} radii="300" aria-label="Close">
                   <Icon src={Icons.Cross} />
                 </IconButton>
               </Header>