fix(school): show Nursery only for nursery classes, and say Girls' school

GIAS NurseryProvision is text ("Has Nursery Classes", "No Nursery
Classes", "Not applicable"), but the header and the place table tested it
for truthiness. Every school with a value got a Nursery chip or a "Yes",
including secondaries aged 11-18. hasNurseryClasses() matches the one
value that means a nursery, and the type now says the field is a string.

The single-sex chip appended 's to the plural GIAS gender, giving
"Girls's school". singleSexLabel() gives "Girls' school" / "Boys' school".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

nextjs-app/__tests__/lib/utils.test.ts

@@ -15,6 +15,8 @@ import {
   computeYBounds,
   formatAgeRange,
   formatAgeSpan,
+  hasNurseryClasses,
+  singleSexLabel,
 } from '@/lib/utils';
 
 describe('formatPercentage', () => {
@@ -346,3 +348,27 @@ describe('formatAgeRange', () => {
     expect(formatAgeRange('4-11')).toBe('Ages 4–11');
   });
 });
+
+describe('hasNurseryClasses', () => {
+  it('is true only for the GIAS value that means it', () => {
+    // GIAS sends text, and two of its three values mean no nursery.
+    expect(hasNurseryClasses('Has Nursery Classes')).toBe(true);
+    expect(hasNurseryClasses('No Nursery Classes')).toBe(false);
+    expect(hasNurseryClasses('Not applicable')).toBe(false);
+    expect(hasNurseryClasses(null)).toBe(false);
+    expect(hasNurseryClasses(undefined)).toBe(false);
+  });
+});
+
+describe('singleSexLabel', () => {
+  it('uses the plural possessive GIAS values need', () => {
+    expect(singleSexLabel('Girls')).toBe("Girls' school");
+    expect(singleSexLabel('Boys')).toBe("Boys' school");
+  });
+
+  it('returns null for a mixed or unknown school', () => {
+    expect(singleSexLabel('Mixed')).toBeNull();
+    expect(singleSexLabel(null)).toBeNull();
+    expect(singleSexLabel(undefined)).toBeNull();
+  });
+});