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/navigation.spec.ts

@@ -0,0 +1,31 @@
+import { test, expect } from '@playwright/test';
+
+test.describe('Navigation', () => {
+  test('clicking a hero nav link navigates to a subpage', async ({ page }) => {
+    await page.goto('/');
+    const heroNav = page.locator('.hero__nav .hero__nav-link').first();
+    const href = await heroNav.getAttribute('href');
+    expect(href).toBeTruthy();
+
+    await heroNav.click();
+    await page.waitForURL(`**${href}`);
+    expect(page.url()).toContain(href);
+  });
+
+  test('header About link navigates to /about', async ({ page }) => {
+    await page.goto('/');
+    const aboutLink = page.locator('.header__nav-link', { hasText: 'About' });
+    await aboutLink.click();
+    await page.waitForURL('**/about');
+    expect(page.url()).toContain('/about');
+  });
+
+  test('header Home link navigates back to /', async ({ page }) => {
+    await page.goto('/about');
+    const homeLink = page.locator('.header__nav-link', { hasText: 'Home' });
+    await homeLink.click();
+    await page.waitForURL('**/');
+    // Accept both "/" and "?..." after root
+    expect(new URL(page.url()).pathname).toBe('/');
+  });
+});