fix(design): rating and live colour tokens, the player's menus over its notices, the info overlay on a light theme (E28)

- F16: --rating (theme-scoped gold, 4.5:1 on the page and its chip), --rating-media, --live/--live-fg,
  --media-chrome replace 14 palette literals; a lint rule refuses palette classes; the theme contract
  measures them.
- The Plex player's notices drew over its open settings/tracks popovers: notices z-base, popovers z-menu.
- The player's info overlay rendered theme tokens on its always-dark panel: on a light theme the score
  (2.6:1), collection years (3.2:1) and cast initials (1.05:1) were unreadable; onDark variants now.
- The lint-rule contract tests share one ESLint with a one-off warm-up (a cold first lint took 8 s,
  past vitest's 5 s).

frontend/src/components/UpNextCard.tsx

@@ -67,7 +67,7 @@ export function UpNextCard({
       role="region"
       aria-label={t("plex.player.upNext.title")}
       onClick={(e) => e.stopPropagation()}
-      className="absolute bottom-24 right-6 z-menu flex w-[22rem] max-w-[calc(100vw-3rem)] gap-3 rounded-2xl border border-white/15 bg-neutral-900/95 p-3.5 text-white shadow-2xl"
+      className="absolute bottom-24 right-6 z-menu flex w-[22rem] max-w-[calc(100vw-3rem)] gap-3 rounded-2xl border border-white/15 bg-media-chrome/95 p-3.5 text-white shadow-2xl"
     >
       <div className="h-[6.5rem] w-[4.6rem] flex-none overflow-hidden rounded-lg bg-white/10">
         {next && (