audit pass 13: context menu arrow key navigation

Add ArrowUp/ArrowDown/Home/End keyboard navigation between context
menu items per WAI-ARIA menu widget specification. Focus wraps at
top and bottom boundaries.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

base.js

@@ -1697,7 +1697,15 @@
       el.setAttribute('tabindex', '0');
       el.innerHTML = `${item.icon ? `<span class="icon">${escHtml(item.icon)}</span>` : '<span class="icon"></span>'}<span>${escHtml(item.label || '')}</span>${item.kbd ? `<kbd>${escHtml(item.kbd)}</kbd>` : ''}`;
       el.addEventListener('click', () => { _ctxHide(); if (item.action) item.action(); });
-      el.addEventListener('keydown', e => { if (e.key === 'Enter' || e.key === ' ') { e.preventDefault(); el.click(); } });
+      el.addEventListener('keydown', e => {
+        const items = Array.from(_ctxMenu.querySelectorAll('[role="menuitem"]'));
+        const idx = items.indexOf(e.currentTarget);
+        if (e.key === 'Enter' || e.key === ' ') { e.preventDefault(); el.click(); }
+        else if (e.key === 'ArrowDown') { e.preventDefault(); (items[idx + 1] || items[0]).focus(); }
+        else if (e.key === 'ArrowUp')   { e.preventDefault(); (items[idx - 1] || items[items.length - 1]).focus(); }
+        else if (e.key === 'Home')      { e.preventDefault(); items[0].focus(); }
+        else if (e.key === 'End')       { e.preventDefault(); items[items.length - 1].focus(); }
+      });
       _ctxMenu.appendChild(el);
     });
     _ctxMenu.classList.add('is-open');