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/ClientRoot.tsx

@@ -69,6 +69,7 @@ function ClientRootOptions({ mx }: { mx?: MatrixClient }) {
       variant="Background"
       fill="None"
       onClick={handleToggle}
+      aria-label="Account options"
     >
       <Icon size="200" src={Icons.VerticalDots} />
       <PopOut