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/ScrollToTop.tsx

@@ -0,0 +1,37 @@
+/**
+ * ScrollToTop — fixed ↑ button that appears after scrolling down.
+ * Smooth-scrolls back to the top when clicked.
+ */
+
+'use client';
+
+import { useState, useEffect } from 'react';
+
+const SHOW_THRESHOLD = 300;
+
+export function ScrollToTop() {
+  const [visible, setVisible] = useState(false);
+
+  useEffect(() => {
+    const onScroll = () => {
+      setVisible(window.scrollY > SHOW_THRESHOLD);
+    };
+    window.addEventListener('scroll', onScroll, { passive: true });
+    return () => window.removeEventListener('scroll', onScroll);
+  }, []);
+
+  const scrollToTop = () => {
+    window.scrollTo({ top: 0, behavior: 'smooth' });
+  };
+
+  return (
+    <button
+      className={`scroll-to-top ${visible ? 'scroll-to-top--visible' : ''}`}
+      onClick={scrollToTop}
+      aria-label="Scroll to top"
+      type="button"
+    >
+      ↑
+    </button>
+  );
+}