fix(desktop): the onboarding welcome step is centred with the wordmark, as the board draws it

Onb-Flow's welcome centres a large wordmark, the headline, one line of body,
the primary button and the note; it was a left-aligned card with the skip
beside the button. The skip stays, now under the button. The board's
import-from-backup link is not drawn: onboarding has no import flow to call.

desktop/frontend-next/src/design/onboarding.css

@@ -13,7 +13,11 @@
 .onb-steps li[data-state="current"] i { border-color: var(--text); background: var(--text); color: var(--surface); }
 .onb-steps li[data-state="done"] i { border-color: transparent; background: color-mix(in srgb, var(--ok) 14%, transparent); color: var(--ok-ink); }
 
-.onb-welcome { display: flex; flex-direction: column; gap: 14px; padding: 18px 0 6px; }
+.onb-welcome { display: flex; flex-direction: column; align-items: center; gap: 14px; padding: 28px 0 10px; text-align: center; }
+.onb-welcome .onb-s { max-width: 440px; }
+.onb-welcome-mark { width: 168px; height: auto; margin-bottom: 14px; }
+.onb-flow .onb-welcome .onb-acts { flex-direction: column; align-items: center; gap: 10px; margin-top: 6px; }
+.onb-flow .onb-welcome .onb-acts .onb-go { min-width: 240px; }
 .onb-welcome .onb-t { font-size: var(--fs-display); }
 
 .onb-services { display: grid; grid-template-columns: repeat(auto-fill, minmax(120px, 1fr)); gap: 8px; }

desktop/frontend-next/src/ui/Onboarding.tsx

@@ -214,6 +214,7 @@ export function Onboarding({ port, setup, onDone, onSkip }: Props) {
 
           {step === "welcome" && (
             <div className="onb-welcome">
+              <img className="onb-welcome-mark" src={logoWordmark} alt="" aria-hidden="true" draggable={false} />
               <h1 className="onb-t">{t("onboarding.hand_it_a_task")}</h1>
               <p className="onb-s">{t("onboarding.what_it_does_and_shows")}</p>
               <div className="onb-acts">