fix(search): offer every phase while a phase filter is applied

The phase select read its options from the result-scoped filters, which
the backend computes after applying the phase filter. With secondary
chosen only secondary and all-through were offered, so switching to
primary meant going back to "Any phase" first. Read the global phase
list instead.

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

nextjs-app/__tests__/components/FilterBarPhase.test.tsx

@@ -0,0 +1,40 @@
+import { render, screen, within } from '@testing-library/react';
+import { FilterBar } from '@/components/FilterBar';
+
+let searchParams = new URLSearchParams();
+jest.mock('next/navigation', () => ({
+  useRouter: () => ({ push: jest.fn(), replace: jest.fn(), prefetch: jest.fn() }),
+  usePathname: () => '/',
+  useSearchParams: () => searchParams,
+}));
+
+const FILTERS = {
+  local_authorities: [], school_types: [], years: [],
+  phases: ['Primary', 'Secondary', 'All-through'],
+  genders: [], admissions_policies: [],
+};
+
+/**
+ * The phase options must not come from the result set. The backend scopes its
+ * result filters to the schools it returns, and it applies the phase filter
+ * first — so with "secondary" chosen the scoped list holds only secondary-ish
+ * phases, and switching to primary meant going back to "Any phase" first.
+ */
+describe('FilterBar phase options', () => {
+  it('offers every phase while a phase filter narrows the results', () => {
+    searchParams = new URLSearchParams('search=hampton&phase=secondary');
+    render(
+      <FilterBar
+        filters={FILTERS}
+        resultFilters={{
+          local_authorities: [], school_types: [],
+          phases: ['Secondary', 'All-through'],
+          genders: [], admissions_policies: [],
+        }}
+      />,
+    );
+    const phase = screen.getByRole('combobox', { name: 'Phase' });
+    expect(within(phase).getByRole('option', { name: 'Primary' })).toBeInTheDocument();
+    expect(phase).toHaveValue('secondary');
+  });
+});