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/DuplicateFinder.tsx

@@ -71,7 +71,7 @@ function DuplicateGroupCard({ group }: { group: DuplicateGroup }) {
           return (
             <button
               key={image.id}
-              className={`group relative overflow-hidden rounded-xl border transition-all ${
+              className={`media-dark-surface group relative overflow-hidden rounded-xl border transition-all ${
                 isSelected
                   ? "border-red-400/50 ring-1 ring-red-400/30"
                   : "border-white/8 hover:border-white/20"