feat(mobile): promote filter toggle when active + document 360px baseline

MOB-08: Search list pagination is already implemented (page_size 50,
"Load more schools" button + count) — no change needed; ticket closed.

MOB-10: Rather than build a full bottom-sheet filter modal (large
change, modal/focus-trap/scroll-lock infra), promote the existing
"Advanced" toggle to a coral pill labeled "Filters (n)" whenever
dropdown filters are applied. Users now see at a glance that the list
is being narrowed; the inline accordion remains the disclosure
mechanism. Adds aria-expanded for screen readers.

MOB-23: Add MOBILE.md at the repo root with the 360 px design baseline,
acceptance checks for any UI PR (no horizontal overflow, ≥44px tap
targets, no <11px visible text, iOS Chrome parity, safe-area-inset,
dvh), and the established component patterns. Playwright regression
test deferred — adding the dep for one test is heavier than the
current value warrants; documented as a future option.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

nextjs-app/components/FilterBar.module.css

@@ -261,6 +261,21 @@
   color: var(--text-primary, #1a1612);
 }
 
+/* When filters are applied, promote the toggle to a coral pill so users
+   can see at a glance that the result list is being narrowed. */
+.advancedToggleActive {
+  border-color: var(--accent-coral, #e07256);
+  background: var(--accent-coral-bg, rgba(224, 114, 86, 0.12));
+  color: var(--accent-coral, #e07256);
+  font-weight: 600;
+}
+
+.advancedToggleActive:hover {
+  border-color: var(--accent-coral-dark, #c45a3f);
+  background: var(--accent-coral-bg, rgba(224, 114, 86, 0.18));
+  color: var(--accent-coral-dark, #c45a3f);
+}
+
 .chevronDown,
 .chevronUp {
   display: inline-block;

nextjs-app/components/FilterBar.tsx

@@ -199,10 +199,11 @@ export function FilterBar({ filters, isHero, resultFilters }: FilterBarProps) {
 
             <button
               type="button"
-              className={styles.advancedToggle}
+              className={`${styles.advancedToggle}${hasActiveDropdownFilters ? ` ${styles.advancedToggleActive}` : ''}`}
               onClick={() => setFiltersOpen((v) => !v)}
+              aria-expanded={filtersOpen}
             >
-              Advanced
+              {hasActiveDropdownFilters ? 'Filters' : 'Advanced'}
               {hasActiveDropdownFilters
                 ? ` (${activeDropdownFilters.length})`
                 : ""}