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/lib/theme.ts

@@ -10,7 +10,7 @@ export const SCHEMES: { id: Scheme; name: string; swatch: string }[] = [
   { id: "midnight", name: "Midnight", swatch: "#6d8cff" },
   { id: "forest", name: "Forest", swatch: "#22d3ee" },
   { id: "slate", name: "Slate", swatch: "#e879f9" },
-  { id: "matrix", name: "Matrix", swatch: "#a3e635" },
+  { id: "matrix", name: "Matrix", swatch: "#86a05a" },
 ];
 
 export interface ThemePrefs {