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

content/gallery.yaml.example

@@ -42,3 +42,10 @@ subpages:
     password: clientpass123  # visitors must enter this to view
     albums:
       - 55555555-5555-5555-5555-555555555555
+
+# Footer — optional minimal footer with social links
+# footer:
+#   name: John Doe
+#   instagram: johndoe
+#   email: hello@example.com
+#   website: https://example.com