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/src/App.tsx

@@ -32,7 +32,6 @@ import Header from "./components/Header";
 import NavSidebar from "./components/NavSidebar";
 import BackToTop from "./components/ui/BackToTop";
 import PageShell from "./components/ui/PageShell";
-import GlassTuner from "./components/GlassTuner";
 import ChatDock from "./components/ChatDock";
 import DebugRecorder from "./components/DebugRecorder";
 import Toaster from "./components/ui/Toaster";
@@ -475,8 +474,6 @@ export default function App() {
       <ErrorDialog />
       {/* Re-binds to the active page's scroll container on navigation (page or channel change). */}
       <BackToTop dep={channelView ? `chan:${channelView.id}` : page} />
-      {/* Opt-in live appearance tuner (Settings → Appearance; off by default). */}
-      <GlassTuner />
     </div>
   );
 }