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/ClientLayout.tsx

@@ -7,9 +7,29 @@ type ClientLayoutProps = {
 };
 export function ClientLayout({ nav, children }: ClientLayoutProps) {
   return (
-    <Box grow="Yes">
-      <Box shrink="No">{nav}</Box>
-      <Box grow="Yes">{children}</Box>
-    </Box>
+    <>
+      <a
+        href="#main-content"
+        style={{
+          position: 'absolute',
+          top: '-40px',
+          left: 0,
+          padding: '8px 16px',
+          background: '#000',
+          color: '#fff',
+          zIndex: 9999,
+          borderRadius: '0 0 4px 0',
+          transition: 'top 0.1s',
+        }}
+        onFocus={(e) => { (e.currentTarget as HTMLElement).style.top = '0'; }}
+        onBlur={(e) => { (e.currentTarget as HTMLElement).style.top = '-40px'; }}
+      >
+        Skip to main content
+      </a>
+      <Box grow="Yes">
+        <Box shrink="No" as="nav" aria-label="Room navigation">{nav}</Box>
+        <Box grow="Yes" as="main" id="main-content">{children}</Box>
+      </Box>
+    </>
   );
 }