feat(ui): g100 dark theme and more space above page titles

- all pages (landing, both tools, server error pages) render on Carbon's
  g100 theme via <body class="app cds--g100">; token consumers follow
  automatically, including component shadow styles through the inherited
  --cds-* custom properties
- dark base on <html> so no light flash around the viewport
- step chips and tool icons use the brand blue: layer-selected-inverse
  inverts to a LIGHT token in g100, which had made them white-on-white
- page titles breathe: .page-head top margin 1.5rem -> 2.75rem
- new test asserting every page renders on g100 (44 total)

tools/server.js

@@ -74,7 +74,7 @@ function errorPage(res, status, title, message, actions = {}) {
   <title>${esc(title)} · YOUreka Tools</title>
   <link rel="stylesheet" href="/assets/style.css">
 </head>
-<body class="app">
+<body class="app cds--g100">
   ${appHeader(null, null)}
   <main class="container narrow center-v">
     <div class="card message-card">