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.

.gitignore

@@ -4,6 +4,7 @@ node_modules/
 **/node_modules/
 dist/
 dist-preview/
+dist-flow-preview/
 packages/paperclip-runner/runner/target/
 ui/storybook-static/
 .env