add skip navigation, page title, and global ARIA live region

src/components/layout/AppShell.tsx

@@ -10,8 +10,14 @@ export function AppShell({ children }: AppShellProps) {
   return (
     <TooltipProvider>
       <div className="flex h-screen flex-col bg-pylon-bg">
+        <a
+          href="#main-content"
+          className="sr-only focus:not-sr-only focus:fixed focus:left-4 focus:top-4 focus:z-[200] focus:rounded-md focus:bg-pylon-accent focus:px-4 focus:py-2 focus:text-sm focus:font-medium focus:text-white focus:shadow-lg"
+        >
+          Skip to main content
+        </a>
         <TopBar />
-        <main className="flex-1 overflow-hidden">{children}</main>
+        <main id="main-content" className="flex-1 overflow-hidden">{children}</main>
       </div>
     </TooltipProvider>
   );