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/public/index.html

@@ -15,7 +15,7 @@
     cds-tile:not(:defined), cds-clickable-tile:not(:defined) { visibility: hidden; }
   </style>
 </head>
-<body class="app">
+<body class="app cds--g100">
   <!--app-header-->
 
   <main class="container">