feat(learn): add standalone /learn pages backed by Page Data

Serve lightweight child/parent learning assistant pages at /learn without
MindSpace injection, with a rebuild script and Portal static route.

Co-authored-by: Cursor <cursoragent@cursor.com>

server/portal-static-delivery-routes.mjs

@@ -161,6 +161,13 @@ export function attachPortalStaticDeliveryRoutes({
       { maxAge: '1d' },
     ),
   );
+  app.use(
+    '/learn',
+    staticMiddleware(
+      pathApi.join(h5Root, 'public/learn'),
+      { maxAge: '5m', index: 'index.html' },
+    ),
+  );
 
   app.get(
     '/dev/wechat-share-preview',