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/sidebar/SpaceTabs.tsx

@@ -522,7 +522,7 @@ function OpenedSpaceFolder({ folder, onClose, children }: OpenedSpaceFolderProps
     >
       <SidebarFolderDropTarget ref={aboveTargetRef} position="Top" />
       <SidebarAvatar size="300">
-        <IconButton data-id={folder.id} size="300" variant="Background" onClick={onClose}>
+        <IconButton data-id={folder.id} size="300" variant="Background" onClick={onClose} aria-label="Close folder">
           <Icon size="400" src={Icons.ChevronTop} filled />
         </IconButton>
       </SidebarAvatar>