fix(player): what is chosen on the player chrome is marked amber, not written in the accent

A light theme's accent as text on the always-dark chrome was 2.84:1 (Slate). `--chosen` is the
accent unless a scope names its own; `media-marks` (was media-focus) points it, with the focus
ring, at the media amber. A chosen track stays white with an amber tick (MenuCheck reads
`text-chosen`); a chosen settings tab or toggle is white with an amber underline (MEDIA_CHOSEN).
The theme contract test holds the token, the tick and the player components free of accent text.

frontend/src/components/ui/MenuCheck.tsx

@@ -13,7 +13,7 @@ export function MenuCheck({ kind, on }: { kind: "many" | "one"; on: boolean | "m
     return (
       <Check
         aria-hidden="true"
-        className={`size-4 shrink-0 ${on === true ? "text-accent" : "opacity-0"}`}
+        className={`size-4 shrink-0 ${on === true ? "text-chosen" : "opacity-0"}`}
       />
     );
   return (