feat(appearance): token-driven theme engine with schemes and FOUC-safe boot

applyAppearance is the single writer of styling to the DOM (the .dark class plus data-scheme/-no-transparency/-density/-reduce-motion and the custom-accent/type-scale CSS vars). An external pre-paint /theme-boot.js replays a cached snapshot before first paint and complies with the production CSP (script-src 'self'), fixing the long-standing theme FOUC. Adds seven color schemes (incl. a true high-contrast that raises neutral contrast), a custom accent with auto-derived legible text, an extended token layer (accent variants, status/shadow/overlay/inverse), a scheme-gated legacy accent bridge, and a transparency-off layer. The default scheme sets no attributes, so existing users are unaffected.

client/index.html

@@ -5,6 +5,10 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no, viewport-fit=cover" />
     <title>TREK</title>
 
+    <!-- Pre-paint appearance (FOUC fix). External classic script so it runs
+         before first paint AND complies with the prod CSP (script-src 'self'). -->
+    <script src="/theme-boot.js"></script>
+
     <!-- PWA / iOS -->
     <meta name="theme-color" content="#09090b" />
     <meta name="apple-mobile-web-app-capable" content="yes" />