feat(web): render similar schools on both detail templates

Inside SchoolDetailShell rather than after it, because the sticky nav's
scroll-spy finds sections with getElementById and can only reach one that
lives in the shell. Last in the order, and last in the nav, because the two
must agree or the nav links to an anchor that was never rendered.

hasSimilarSchools is optional on NavItemsInput, matching hasLocation beside
it: absent has to mean "no section", and making it required would have
churned ten unrelated call sites for no added safety.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nextjs-app/app/(frontend)/school/[slug]/page.tsx

@@ -8,6 +8,7 @@ import { APIFetchError, fetchSchoolDetails, fetchSchools, fetchNationalAverages
 import { notFound, redirect } from 'next/navigation';
 import { SchoolDetailShell } from '@/components/school/SchoolDetailShell';
 import { NearbyPlaces } from '@/components/school/NearbyPlaces';
+import { shouldRenderSimilar } from '@/components/school/SimilarSchoolsSection';
 import { schoolBreadcrumbJsonLd, type SchoolPlace } from '@/lib/jsonld';
 import { PrimarySchoolSections } from '@/components/school/PrimarySchoolSections';
 import { SecondarySchoolSections } from '@/components/school/SecondarySchoolSections';
@@ -155,6 +156,8 @@ export default async function SchoolPage({ params }: SchoolPageProps) {
   // nothing rather than throwing, which is how this shipped without a
   // lockstep deploy of the two images.
   const places: SchoolPlace[] = data.places ?? [];
+  // Absent on an older API build, exactly like `places` above.
+  const similarSchools = data.similar_schools ?? [];
 
   // Redirect bare URN to canonical slug URL
   const canonicalSlug = schoolUrl(urn, school_info.school_name).replace('/school/', '');
@@ -186,6 +189,7 @@ export default async function SchoolPage({ params }: SchoolPageProps) {
     admissions: admissions ?? null,
     admissionDistance: admission_distance ?? null,
     hasLocation: school_info.latitude != null && school_info.longitude != null,
+    hasSimilarSchools: shouldRenderSimilar(similarSchools),
     yearlyDataLength: yearly_data.length,
   };
   const primaryNavItems = buildNavItems(primaryFlags, navInput);
@@ -262,6 +266,7 @@ export default async function SchoolPage({ params }: SchoolPageProps) {
             finance={finance ?? null}
             nationalAvg={nationalAvg}
             destinations={destinations ?? null}
+            similarSchools={similarSchools}
             flags={secondaryFlags}
           />
         </SchoolDetailShell>
@@ -284,6 +289,7 @@ export default async function SchoolPage({ params }: SchoolPageProps) {
             deprivation={deprivation ?? null}
             finance={finance ?? null}
             nationalAvg={nationalAvg}
+            similarSchools={similarSchools}
             flags={primaryFlags}
           />
         </SchoolDetailShell>