feat(design): Matrix toned down to moss, text on an accent fill held to --accent-fg

The lime accent was the loudest of the four (OKLCH L 0.85, C 0.21) and read as neon; the owner chose the
moss variant at Midnight's lightness with near-neutral greys, and the Matrix backdrops follow it. Fixed
white/black parts on a solid accent fill (two switch knobs, three play buttons, the Up next pill) now use
--accent-fg, and theme.contract.test.ts refuses a fixed white or black next to a solid bg-accent.

frontend/src/components/PlexPlaylistView.tsx

@@ -569,7 +569,7 @@ export default function PlexPlaylistView({
           {items.length > 0 && (
             <button
               onClick={() => onPlay(order[0]!, order)}
-              className="inline-flex items-center gap-2 rounded-xl bg-accent px-4 py-2 font-medium text-white hover:opacity-90"
+              className="inline-flex items-center gap-2 rounded-xl bg-accent px-4 py-2 font-medium text-accent-fg hover:opacity-90"
             >
               <Play className="h-5 w-5 fill-current" />
               {t("plex.playlist.playAll")}