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/lib/schoolSections.ts

@@ -128,6 +128,10 @@ export interface NavItemsInput {
    *  measure a postcode, so the nav must gate on them too or it will link to an
    *  anchor that was never rendered. */
   hasLocation?: boolean;
+  /** Whether the similar-schools section will render. Optional for the same
+   *  reason hasLocation is: the nav must never link to an anchor that was not
+   *  rendered, and absent has to mean "no section". */
+  hasSimilarSchools?: boolean;
   yearlyDataLength: number;
 }
 
@@ -142,7 +146,10 @@ export interface NavItemsInput {
  */
 export function buildNavItems(
   flags: SchoolFlags,
-  { ofsted, admissions, admissionDistance, hasLocation, yearlyDataLength }: NavItemsInput,
+  {
+    ofsted, admissions, admissionDistance, hasLocation,
+    hasSimilarSchools, yearlyDataLength,
+  }: NavItemsInput,
 ): NavItem[] {
   const navItems: NavItem[] = [];
   if (ofsted) navItems.push({ id: 'ofsted', label: 'Ofsted' });
@@ -161,6 +168,8 @@ export function buildNavItems(
   if (flags.hasSchoolLife) navItems.push({ id: 'school-life', label: 'School Life' });
   if (flags.hasDeprivation) navItems.push({ id: 'local-area', label: 'Local Area' });
   if (flags.hasFinance) navItems.push({ id: 'finances', label: 'Finances' });
+  // Last, because the section renders last — the scroll-spy reads this order.
+  if (hasSimilarSchools) navItems.push({ id: 'similar', label: 'Similar schools' });
   return navItems;
 }
 
@@ -239,7 +248,10 @@ export function computeSecondaryFlags({
  */
 export function buildSecondaryNavItems(
   flags: SecondaryFlags,
-  { ofsted, admissions, admissionDistance, hasLocation, yearlyDataLength }: NavItemsInput,
+  {
+    ofsted, admissions, admissionDistance, hasLocation,
+    hasSimilarSchools, yearlyDataLength,
+  }: NavItemsInput,
 ): NavItem[] {
   const navItems: NavItem[] = [];
   if (ofsted) navItems.push({ id: 'ofsted', label: 'Ofsted' });
@@ -257,5 +269,7 @@ export function buildSecondaryNavItems(
   if (yearlyDataLength > 1) navItems.push({ id: 'history', label: 'History' });
   if (flags.hasWellbeing) navItems.push({ id: 'wellbeing', label: 'Wellbeing' });
   if (flags.hasFinance) navItems.push({ id: 'finances', label: 'Finances' });
+  // Last, because the section renders last — the scroll-spy reads this order.
+  if (hasSimilarSchools) navItems.push({ id: 'similar', label: 'Similar schools' });
   return navItems;
 }