fix(map): results map fullscreen falls back to an overlay on iOS

The results-view map's fullscreen button called requestFullscreen(),
which iOS Safari doesn't implement (fullscreen is video-only there), so
tapping it did nothing on iPhones — the same gap already fixed for the
school hero map.

When the Fullscreen API is missing or its promise rejects, fall back to
a fixed-position overlay (.fsFallback, z-index 5000) driven by state,
locking body scroll while open. Leaflet re-measures on window resize, so
dispatch a resize when fullscreen toggles (the CSS overlay fires none) or
the map would fill only part of the screen. Native fullscreen is
unchanged.

New e2e journey deletes Element.requestFullscreen on a mobile viewport,
opens the results map fullscreen, and asserts the exit control appears
then releases; it fails against current production, reproducing the bug.

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

e2e/tests/journeys.spec.ts

@@ -75,6 +75,31 @@ test('school hero map opens fullscreen on mobile without the Fullscreen API', as
   await expect(openMap).toBeVisible();
 });
 
+test('results map fullscreen falls back to an overlay on iOS', async ({ page }) => {
+  // Same iOS gap as the hero map: no Element.requestFullscreen, so the results
+  // map's fullscreen button must fall back to a CSS overlay.
+  await page.setViewportSize({ width: 390, height: 844 });
+  await page.addInitScript(() => {
+    // @ts-expect-error deliberate API removal
+    delete Element.prototype.requestFullscreen;
+  });
+
+  await searchByName(page, 'B1 1BB');
+  await expect(schoolLinks(page).first()).toBeVisible({ timeout: 15_000 });
+
+  // Switch to the map view, then open the map fullscreen.
+  await page.getByRole('button', { name: 'Map', exact: true }).click();
+  const openFs = page.getByRole('button', { name: 'View map fullscreen' });
+  await expect(openFs).toBeVisible({ timeout: 15_000 });
+  await openFs.click();
+
+  // The button flips to its exit state once the overlay is up.
+  const exitFs = page.getByRole('button', { name: 'Exit fullscreen' });
+  await expect(exitFs).toBeVisible();
+  await exitFs.click();
+  await expect(openFs).toBeVisible();
+});
+
 test('comparing two schools shows both side by side', async ({ page }) => {
   // Collect two school URNs from search results, then load the share URL
   await searchByName(page, 'primary');