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

lib/__tests__/config.test.ts

@@ -0,0 +1,53 @@
+import { describe, it, expect, vi } from 'vitest';
+
+// Mock env.ts so importing config.ts doesn't trigger Zod validation
+vi.mock('@/lib/env', () => ({
+  env: {
+    IMMICH_API_URL: 'http://localhost:2283',
+    IMMICH_API_KEY: 'test-key',
+    SITE_TITLE: 'Test Gallery',
+    SITE_SUBTITLE: '',
+    CACHE_TTL: 300,
+    RATE_LIMIT_RPM: 120,
+  },
+}));
+
+import { slugify } from '@/lib/config';
+
+describe('slugify', () => {
+  it('converts a simple name to lowercase with hyphens', () => {
+    expect(slugify('Kloster Chorin')).toBe('kloster-chorin');
+  });
+
+  it('strips diacritics', () => {
+    expect(slugify('Schöne Aussicht')).toBe('schone-aussicht');
+    expect(slugify('Café Müller')).toBe('cafe-muller');
+  });
+
+  it('replaces special characters with hyphens', () => {
+    expect(slugify('Hello, World!')).toBe('hello-world');
+    expect(slugify('foo & bar (baz)')).toBe('foo-bar-baz');
+  });
+
+  it('trims leading and trailing hyphens', () => {
+    expect(slugify('--leading')).toBe('leading');
+    expect(slugify('trailing---')).toBe('trailing');
+    expect(slugify('---both---')).toBe('both');
+  });
+
+  it('collapses consecutive hyphens', () => {
+    expect(slugify('a   b   c')).toBe('a-b-c');
+  });
+
+  it('handles an empty string', () => {
+    expect(slugify('')).toBe('');
+  });
+
+  it('handles all-special-character input', () => {
+    expect(slugify('!@#$%')).toBe('');
+  });
+
+  it('preserves numbers', () => {
+    expect(slugify('Album 2024')).toBe('album-2024');
+  });
+});