feat: visual polish & UX — fade-in, theme toggle, scroll-to-top, footer, cursor, badge

- Add FadeIn component (IntersectionObserver scroll-reveal + load animation)
- Add dark/light theme toggle (useSyncExternalStore + localStorage)
- Add scroll-to-top button (appears after 300px scroll)
- Add configurable footer with social links (gallery.yaml)
- Add crosshair cursor on photo grid items
- Add 'X photos' count badge on subpage cover images
- Add light theme color palette in tokens.css

e2e/health.spec.ts

@@ -0,0 +1,10 @@
+import { test, expect } from '@playwright/test';
+
+test.describe('Health API', () => {
+  test('GET /api/health returns 200 with expected shape', async ({ request }) => {
+    const response = await request.get('/api/health');
+    expect(response.ok()).toBeTruthy();
+    const body = await response.json();
+    expect(body).toHaveProperty('status');
+  });
+});