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__/tokens.test.ts

@@ -0,0 +1,62 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+// Mock the config module so tokens.ts can resolve getConfig().immich.apiKey
+vi.mock('@/lib/config', () => ({
+  getConfig: () => ({
+    immich: { apiKey: 'test-api-key-for-vitest-unit-tests' },
+  }),
+}));
+
+import { encodeAssetId, decodeAssetId } from '@/lib/tokens';
+
+const SAMPLE_UUID = '550e8400-e29b-41d4-a716-446655440000';
+
+describe('tokens', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  describe('encodeAssetId', () => {
+    it('produces a non-empty base64url string', () => {
+      const token = encodeAssetId(SAMPLE_UUID);
+      expect(token).toBeTruthy();
+      expect(typeof token).toBe('string');
+      // base64url characters only
+      expect(token).toMatch(/^[A-Za-z0-9_-]+$/);
+    });
+
+    it('is deterministic — same input yields same token', () => {
+      const t1 = encodeAssetId(SAMPLE_UUID);
+      const t2 = encodeAssetId(SAMPLE_UUID);
+      expect(t1).toBe(t2);
+    });
+
+    it('produces different tokens for different UUIDs', () => {
+      const t1 = encodeAssetId(SAMPLE_UUID);
+      const t2 = encodeAssetId('12345678-aaaa-bbbb-cccc-123456789abc');
+      expect(t1).not.toBe(t2);
+    });
+  });
+
+  describe('decodeAssetId', () => {
+    it('round-trips a valid UUID', () => {
+      const token = encodeAssetId(SAMPLE_UUID);
+      const decoded = decodeAssetId(token);
+      expect(decoded).toBe(SAMPLE_UUID);
+    });
+
+    it('returns null for a garbage string', () => {
+      expect(decodeAssetId('not-a-valid-token')).toBeNull();
+    });
+
+    it('returns null for an empty string', () => {
+      expect(decodeAssetId('')).toBeNull();
+    });
+
+    it('returns null for a token that is too short', () => {
+      // less than 17 bytes when decoded
+      const short = Buffer.from('shortdata').toString('base64url');
+      expect(decodeAssetId(short)).toBeNull();
+    });
+  });
+});