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/plexDetailUi.tsx

@@ -144,7 +144,7 @@ export function PrefToggle({
         className={`relative h-4 w-7 rounded-full transition ${on ? "bg-accent" : "bg-border"}`}
       >
         <span
-          className={`absolute top-0.5 h-3 w-3 rounded-full bg-white transition-all ${on ? "left-3.5" : "left-0.5"}`}
+          className={`absolute top-0.5 h-3 w-3 rounded-full transition-all ${on ? "left-3.5 bg-accent-fg" : "left-0.5 bg-white"}`}
         />
       </span>
     </button>