[Polish] AI Credits Setup onboarding — copy, UX, and UI finalization

[blueogin]

Final polish pass on the Setup onboarding flow (guidance card, Download AntSeed, Signer Key,
Authorize Wallet). No new functionality.

Scope

• Copy — consistent terminology (Signer Key / Authorize Wallet); Manage still says
  "Buyer & Operator". Review status and error wording, and the Authorize Wallet
  permission copy. Verify outbound links.
• UX — step gating, drawer/back behaviour, loading and failure feedback, no dead ends,
  clear handoff to the Buy tab.
• UI — design-system props instead of ad-hoc styling (Anchor style in
  HowToUseView.tsx:73,115 and SetupGuidanceCard.tsx:108; invalid variant="default" in
  SetupGuidanceCard.tsx:95,120), light/dark and mobile-width checks.
• Edge cases — the three operator states, plus a failed operator-status fetch after import.
• QA — a story per Setup state; test:storybook passes.