Add app-shell heading and skip link (WCAG 2.4.1 / GOV.UK)

src/app/App.tsx

@@ -23,14 +23,20 @@ export function App() {
 
   return (
     <div className={styles.app}>
+      {/* Skip link (WCAG 2.4.1 / GOV.UK): the first focusable element, hidden
+          until focused, lets keyboard users bypass the header into the work area. */}
+      <a className={styles.skipLink} href="#main">
+        Skip to content
+      </a>
       <header className={styles.header}>
-        <span className={styles.title}>Astrolabe</span>
+        <h1 className={styles.title}>Astrolabe</h1>
         <span className={styles.version}>v{__APP_VERSION__}</span>
         <span className={styles.spacer} />
         <ThemeToggle />
       </header>
 
-      <main className={styles.panes}>
+      {/* tabIndex -1 makes the landmark a focus target for the skip link. */}
+      <main id="main" className={styles.panes} tabIndex={-1}>
         <section
           id="pane-library"
           className={styles.pane}