feat(onboarding): the connect step's sign-in as one continuous sequence (#12863)

Picking a source starts the sign-in: the row collapses to the answer, the card opens where the credential link was, and the footer button walks Sign in -> Waiting for code -> Connecting before the step advances. Back unwinds it a beat at a time.

Nothing mounts to change layout - the card and the link are always rendered and their heights animate, with inert holding the a11y line - because a mount changes the page in one frame and no easing can smooth a step already taken.

Review fixes in the same branch: the displayed-code panel now reports its prompt upward (the OpenAI path could not leave the loading beat without it), the two-second hold is a cancellable beat rather than a dropped timer, unwinding a sequence that never opened a card no longer starts a login to cancel it, and the key field regains focus-on-open.

ui/wizard-preview.html

@@ -0,0 +1,28 @@
+<!DOCTYPE html>
+<html lang="en" class="dark" style="color-scheme: dark">
+  <head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0, viewport-fit=cover" />
+    <meta name="theme-color" content="#18181b" />
+    <title>Connect step — the real wizard</title>
+    <link rel="icon" href="/favicon.svg" type="image/svg+xml" />
+    <style>
+      /*
+        Unlayered, for the same reason `connect-model-preview.html` carries the
+        same rule: index.css pins `html, body` to viewport height with
+        `overflow: hidden` inside @layer base, which is right for the app shell
+        and clips this page's footer out of reach on a short window.
+      */
+      html,
+      body {
+        height: auto;
+        min-height: 100%;
+        overflow-y: auto;
+      }
+    </style>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script type="module" src="/src/wizard-preview-main.tsx"></script>
+  </body>
+</html>