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/ui/form.tsx

@@ -43,8 +43,8 @@ export function Switch({
       className={`w-9 h-5 rounded-full transition relative shrink-0 disabled:opacity-50 disabled:cursor-not-allowed ${checked ? "bg-accent" : "bg-border"}`}
     >
       <span
-        className={`absolute top-0.5 w-4 h-4 rounded-full bg-white shadow transition-all ${
-          checked ? "left-4.5" : "left-0.5"
+        className={`absolute top-0.5 w-4 h-4 rounded-full shadow transition-all ${
+          checked ? "left-4.5 bg-accent-fg" : "left-0.5 bg-white"
         }`}
       />
     </FocusSafeButton>