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/test/server.test.js

@@ -312,6 +312,22 @@ test('injected session values cannot break out of the script tag', async () => {
   }
 });
 
+test('all pages render on the g100 dark theme', async () => {
+  const { base, stop } = await bootApp();
+  try {
+    const cookies = await login(base);
+    for (const path of ['/', '/fingerprint']) {
+      const html = await (await fetch(`${base}${path}`, { headers: { cookie: cookies } })).text();
+      assert.match(html, /<body class="app cds--g100">/, `${path} uses g100`);
+    }
+    // error pages too
+    const errorHtml = await (await fetch(`${base}/auth/callback?sessionId=nope`, manual)).text();
+    assert.match(errorHtml, /<body class="app cds--g100">/);
+  } finally {
+    await stop();
+  }
+});
+
 test('landing header is server-rendered for the session', async () => {
   const { base, stop } = await bootApp();
   try {