Add AntSeed VPR/Desktop download step to AI Credits Setup onboarding

[Copilot]

This updates the AI Credits Setup tab to make Download AntSeed the first onboarding step and surface the outbound VPR/Desktop download action directly in Setup. The Buy deposit/stream flow remains separate, and existing Manage/History behavior is untouched.

• Setup onboarding shell

  • Replaced the static Setup step list with a dedicated onboarding panel built around the existing Stepper primitive.
  • Preserved the required order: Download AntSeed → Signer Key → Authorize Wallet.
  • Kept Signer Key and Authorize Wallet visually locked behind the first setup step for this slice.

• Download AntSeed action

  • Added widget-local Setup content for the first step with a prominent external CTA:
    • Download AntSeed VPR/Desktop
  • Wired the CTA to the stable AntSeed latest-release target:
    • https://github.com/AntSeed/antseed/releases/latest

• Storybook / reviewer coverage

  • Updated the AI Credits widget docs copy to reflect that Setup now starts with the AntSeed download action.
  • Added a QA-story interaction assertion for the Setup tab link target.

• Playwright coverage

  • Extended the existing Setup-tab smoke coverage to assert:
    • Download AntSeed is present
    • step order remains correct
    • the outbound link resolves to the expected AntSeed release URL

const SETUP_ONBOARDING_STEPS: StepperStepItem[] = [
  { id: 'download-antseed', title: 'Download AntSeed', status: 'ready' },
  { id: 'signer-key', title: 'Signer Key', status: 'pending' },
  { id: 'authorize-wallet', title: 'Authorize Wallet', status: 'pending' },
]

<Anchor href="https://github.com/AntSeed/antseed/releases/latest" target="_blank">
  <Button>
    <ButtonText>Download AntSeed VPR/Desktop</ButtonText>
  </Button>
</Anchor>

• Fixes [PLAN] AI Credits onboarding: Download AntSeed step content #197

[Copilot]

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