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/index.css

@@ -960,24 +960,26 @@ html[data-scheme="slate"][data-theme="light"] {
 }
 
 /* Matrix — near-black with a lime accent (E28 B2: the terminal green read as success) */
+/* Matrix is moss, not neon (owner, 2026-09-24): the accent sits at Midnight's lightness (OKLCH L 0.67)
+   with half the old lime's chroma, and the neutrals are barely tinted, so the page doesn't glow green. */
 html[data-scheme="matrix"][data-theme="dark"] {
-  --bg: #030a06;
-  --surface: #08130d;
-  --card: #0b1c12;
-  --border: #163d27;
-  --fg: #cfe8d6;
-  --muted: #6fae86;
-  --accent: #a3e635;
-  --accent-fg: #04170c;
+  --bg: #0b0e0b;
+  --surface: #111510;
+  --card: #161b15;
+  --border: #2a3226;
+  --fg: #d9ded6;
+  --muted: #8f9a8a;
+  --accent: #86a05a;
+  --accent-fg: #0c1208;
 }
 html[data-scheme="matrix"][data-theme="light"] {
-  --bg: #f0f7f1;
+  --bg: #f4f5f0;
   --surface: #ffffff;
   --card: #ffffff;
-  --border: #cfe6d6;
-  --fg: #08130b;
-  --muted: #4c6b56;
-  --accent: #4d7c0f;
+  --border: #dfe3d8;
+  --fg: #161a14;
+  --muted: #5a6352;
+  --accent: #556b2f;
   --accent-fg: #ffffff;
 }