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/Lightbox.tsx

@@ -496,7 +496,7 @@ export function Lightbox() {
                 </AnimatePresence>
               </div>
 
-              <div className="flex w-72 shrink-0 flex-col border-l border-white/5 bg-gray-900/95">
+              <div className="lightbox-panel flex w-72 shrink-0 flex-col border-l border-white/5 bg-gray-900/95">
                 <div className="flex shrink-0 items-center justify-between px-5 pt-5 pb-4">
                   <div className="min-w-0">
                     <p className="truncate text-sm font-medium text-white">{selectedImage.filename}</p>