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>

e2e/tests/journeys.spec.ts

@@ -272,6 +272,17 @@ test('searching by postcode returns nearby schools', async ({ page }) => {
   await expect(schoolLinks(page).first()).toBeVisible({ timeout: 15_000 });
 });
 
+test('the phase filter switches straight from secondary to primary', async ({ page }) => {
+  // The phase options once came from the result set, which the phase filter
+  // had already narrowed — so with secondary chosen, primary was not offered.
+  await page.goto('/?search=school&phase=secondary');
+  const phase = page.getByRole('combobox', { name: 'Phase' });
+  await expect(phase).toHaveValue('secondary', { timeout: 15_000 });
+  await phase.selectOption('primary');
+  await expect(page).toHaveURL(/[?&]phase=primary(&|$)/);
+  await expect(phase).toHaveValue('primary');
+});
+
 test('a report-card school shows a Report Card badge in search results, not its old grade', async ({ page }) => {
   // List/map badges keyed off ofsted_grade (the carried-forward legacy grade)
   // and never reached the report-card branch, so report-card schools were