fix: subtle-light theme parity for lightbox panel and media surfaces

In subtle-light, the lightbox dragged its whole surface (including the metadata
panel) dark via media-dark-surface, while conventional-dark themed the panel
normally. Scope the dark surface to the media canvas and re-light the panel by
remapping --color-* variables on a .lightbox-panel wrapper (Tailwind v4 resolves
every colour utility through these vars, so this re-themes the subtree —
including accents — with no !important). Mark gallery/duplicate media tiles as
media-dark-surface so their on-image overlays stay light-on-dark, and theme the
window restore icon via var(--color-gray-950) instead of a hardcoded hex.

src/components/Gallery.tsx

@@ -124,7 +124,7 @@ export function ImageTile({
 
   return (
     <button
-      className="group relative overflow-hidden rounded-xl bg-white/[0.04] text-left focus:outline-none"
+      className="media-dark-surface group relative overflow-hidden rounded-xl bg-white/[0.04] text-left focus:outline-none"
       style={{ width: "100%", aspectRatio: "1 / 1" }}
       onClick={onClick}
       onContextMenu={onContextMenu}