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/UpNextCard.tsx

@@ -86,7 +86,7 @@ export function UpNextCard({
         <div className="mt-auto flex items-center gap-2 pt-2.5">
           <button
             onClick={onAdvance}
-            className="flex items-center gap-1.5 rounded-full bg-accent px-3 py-1.5 text-xs font-semibold text-black hover:brightness-110"
+            className="flex items-center gap-1.5 rounded-full bg-accent px-3 py-1.5 text-xs font-semibold text-accent-fg hover:brightness-110"
           >
             <span className="relative grid h-5.5 w-5.5 flex-none place-items-center">
               <svg viewBox="0 0 24 24" className="absolute inset-0 -rotate-90">