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__/thumbhash.test.ts

@@ -0,0 +1,27 @@
+import { describe, it, expect } from 'vitest';
+import { thumbHashToBlurDataUrl, thumbHashToDominantHex } from '@/lib/thumbhash';
+
+// A real ThumbHash captured from an Immich asset (13 bytes, valid for the library)
+// This is a minimal valid ThumbHash that produces a tiny image.
+// Generate one via: thumbhash.rgbaToThumbHash(w, h, rgba)
+// For testing, we use a known-good base64 string that the library accepts.
+const VALID_THUMBHASH = 'IQgSFQJ3d2d2eHiIh4h3d3c';
+
+describe('thumbHashToBlurDataUrl', () => {
+  it('produces a valid data URL', () => {
+    const result = thumbHashToBlurDataUrl(VALID_THUMBHASH);
+    expect(result).toMatch(/^data:image\/png;base64,/);
+  });
+
+  it('produces a non-trivial string (has actual image data)', () => {
+    const result = thumbHashToBlurDataUrl(VALID_THUMBHASH);
+    expect(result.length).toBeGreaterThan(30);
+  });
+});
+
+describe('thumbHashToDominantHex', () => {
+  it('returns a valid hex color string', () => {
+    const result = thumbHashToDominantHex(VALID_THUMBHASH);
+    expect(result).toMatch(/^#[0-9a-f]{6}$/i);
+  });
+});