feat(search): open postcode searches on the map on phones too

Trial the map as the default for every postcode search, not only on
desktop (DEFAULT_LOCATION_VIEW in HomeView; results_view_changed shows
how often readers switch away). The view is now derived: the reader's
choice once made, else the default for the kind of search. That follows
a hero search through client-side navigation without syncing state.

With nothing left varying by device, the user-agent default goes
(lib/device.ts, the headers() read in page.tsx). That settles two review
points: the HTML no longer needs Vary: User-Agent, and there is no
narrow-window fallback left to flash the map before the list.

Review fix in LeafletMapInner: the card reopened only when the selection
or the school list changed, so a rebuild caused by the centre, reference
point or radius closed it for good while the page still had the school
selected. The selection effect now follows a pins-rebuilt counter.
SchoolMap memoises its fallback centre, which was a fresh array on every
render and would have rebuilt every pin each time.

Phones hide the map view's list pane, so it no longer builds up to 500
hidden cards there (decided after mount, so hydration still matches).

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

nextjs-app/__tests__/components/HomeView.staleFetch.test.tsx

@@ -39,15 +39,17 @@ beforeEach(() => {
 });
 
 test('load-more results from an old search are discarded, even after returning to it', async () => {
+  // Name searches: a postcode search opens on the map, which has no Load more.
+  params = new URLSearchParams('search=abbey');
   const pending = deferred();
   jest.mocked(fetchSchools).mockReturnValueOnce(pending.promise);
   const view = render(<HomeView initialSchools={response('Initial A')} filters={filters} />);
   fireEvent.click(screen.getByRole('button', { name: 'Load more schools' }));
   const signal = jest.mocked(fetchSchools).mock.calls[0][1]?.signal;
-  params = new URLSearchParams('postcode=SW2+1AA');
+  params = new URLSearchParams('search=brecknock');
   view.rerender(<HomeView initialSchools={response('Initial B')} filters={filters} />);
   expect(signal?.aborted).toBe(true);
-  params = new URLSearchParams('postcode=SW1A+1AA');
+  params = new URLSearchParams('search=abbey');
   view.rerender(<HomeView initialSchools={response('Fresh A')} filters={filters} />);
   await act(async () => pending.resolve(response('Stale append')));
   expect(screen.queryByText('Stale append')).not.toBeInTheDocument();