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

components/SubpageNav.tsx

@@ -7,31 +7,23 @@ import Link from 'next/link';
 import { immich } from '@/lib/immich';
 
 export async function SubpageNav() {
-    const [subpages, standaloneAlbums] = await Promise.all([
-        immich.getSubpages(),
-        immich.getStandaloneAlbums(),
-    ]);
+  const [subpages, standaloneAlbums] = await Promise.all([
+    immich.getSubpages(),
+    immich.getStandaloneAlbums(),
+  ]);
 
-    return (
-        <>
-            {subpages.map((sp) => (
-                <Link
-                    key={sp.slug}
-                    href={`/${sp.slug}`}
-                    className="header__nav-link"
-                >
-                    {sp.name}
-                </Link>
-            ))}
-            {standaloneAlbums.map((album) => (
-                <Link
-                    key={album.id}
-                    href={`/${album.slug}`}
-                    className="header__nav-link"
-                >
-                    {album.albumName}
-                </Link>
-            ))}
-        </>
-    );
+  return (
+    <>
+      {subpages.map((sp) => (
+        <Link key={sp.slug} href={`/${sp.slug}`} className="header__nav-link">
+          {sp.name}
+        </Link>
+      ))}
+      {standaloneAlbums.map((album) => (
+        <Link key={album.id} href={`/${album.slug}`} className="header__nav-link">
+          {album.albumName}
+        </Link>
+      ))}
+    </>
+  );
 }