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/plexCards.tsx

@@ -140,7 +140,7 @@ export function PlexPosterCard({
           <div className="absolute inset-0 bg-black/25" />
           <div className="absolute inset-x-0 top-[30%] flex justify-center">
             <div
-              className="grid h-12 w-12 scale-90 place-items-center rounded-full text-white shadow-xl transition-transform duration-200 group-hover:scale-100"
+              className="grid h-12 w-12 scale-90 place-items-center rounded-full text-accent-fg shadow-xl transition-transform duration-200 group-hover:scale-100"
               style={{
                 background:
                   "radial-gradient(circle at 38% 32%, color-mix(in srgb, var(--accent) 72%, white), var(--accent))",