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/Space.tsx

@@ -273,7 +273,7 @@ function SpaceHeader() {
             {joinRules?.join_rule !== JoinRule.Public && <Icon src={Icons.Lock} size="50" />}
           </Box>
           <Box shrink="No">
-            <IconButton aria-pressed={!!menuAnchor} variant="Background" onClick={handleOpenMenu}>
+            <IconButton aria-pressed={!!menuAnchor} variant="Background" onClick={handleOpenMenu} aria-label="Space options">
               <Icon src={Icons.VerticalDots} size="200" />
             </IconButton>
           </Box>