feat(design): the scheme set, the colour contract and the token utilities (E28 S3a)

Four schemes (Midnight; Forest cyan, Slate fuchsia, Matrix lime), YouTube
and Starship retired with no data migration: normalizeTheme maps a stored
retired scheme to Midnight. A vitest contract measures every scheme x mode
(text 4.5:1, control edge 3:1, status text, the accent's 30-degree hue gap);
it caught light-mode danger at 4.43:1, now red-700. The state layers, the
strong edge, the touch target and the fades are index.css tokens and
utilities the lab now reads. Backdrops recoloured to the new accents, with
a grey overlay grain that actually breaks the banding (longest flat run
15-26px -> 4-8px). The glass tuner is retired.

frontend/public/backdrops/midnight.svg

@@ -5,7 +5,7 @@
     <radialGradient id="c" cx="68%" cy="92%" r="60%"><stop offset="0%" stop-color="#3b2f80" stop-opacity=".9"/><stop offset="100%" stop-color="#3b2f80" stop-opacity="0"/></radialGradient>
     <radialGradient id="d" cx="8%" cy="88%" r="55%"><stop offset="0%" stop-color="#145e6b" stop-opacity=".85"/><stop offset="100%" stop-color="#145e6b" stop-opacity="0"/></radialGradient>
     <filter id="s"><feGaussianBlur stdDeviation="60"/></filter>
-    <filter id="g"><feTurbulence type="fractalNoise" baseFrequency="0.85" numOctaves="2" stitchTiles="stitch"/><feColorMatrix type="matrix" values="0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 .5 0"/></filter>
+    <filter id="g"><feTurbulence type="fractalNoise" baseFrequency="0.85" numOctaves="2" stitchTiles="stitch"/><feColorMatrix type="saturate" values="0"/></filter>
   </defs>
   <rect width="1600" height="1000" fill="#070b16"/>
   <g filter="url(#s)">
@@ -14,5 +14,5 @@
     <rect width="1600" height="1000" fill="url(#d)"/>
     <rect width="1600" height="1000" fill="url(#b)"/>
   </g>
-  <rect width="1600" height="1000" filter="url(#g)" opacity="0.05"/>
+  <rect width="1600" height="1000" filter="url(#g)" opacity="0.35" style="mix-blend-mode:overlay"/>
 </svg>