feat(a11y): add skip link for improved navigation accessibility

libs/ui/src/lib/dev/ThemeGallery.tsx

@@ -22,6 +22,7 @@ import {
   Title,
   useMantineTheme,
 } from '@mantine/core';
+import { SkipLink, MAIN_CONTENT_ID } from '../layout/SkipLink';
 
 /**
  * Every primitive the theme controls, on one page.
@@ -90,7 +91,11 @@ export function ThemeGallery() {
   );
 
   return (
-    <Box p="xl" style={{ maxWidth: 1100, margin: '0 auto' }}>
+    <>
+      {/* Mirrors the real app shells, so the skip-link contract is testable
+          without a session. */}
+      <SkipLink />
+    <Box id={MAIN_CONTENT_ID} p="xl" style={{ maxWidth: 1100, margin: '0 auto' }}>
       <Stack gap="xl">
         <Stack gap={4}>
           <Title order={1}>Theme Gallery</Title>
@@ -308,5 +313,6 @@ export function ThemeGallery() {
         </Section>
       </Stack>
     </Box>
+    </>
   );
 }