feat(ai-credits-widget): Add Download AntSeed as first Setup onboarding step

[Copilot]

Adds the Download AntSeed step as the first actionable step in the Setup tab, replacing the previous generic static card list with a purpose-built component that surfaces the AntSeed VPR/Desktop download link and enforces step ordering visually.

Changes

New component: DownloadAntSeedStep

• Step 1 (active): "Download AntSeed" card with a Start › link to https://antseed.com/download (target="_blank", data-testid="download-antseed-link")
• Steps 2 & 3 (locked): "Signer Key" and "Authorize Wallet" rendered via a shared LockedStep sub-component (greyed out with 🔒), communicating the required sequential ordering

<DownloadAntSeedStep />
// Step 1 — Download AntSeed  [Start ›]  ← external link to antseed.com/download
// Step 2 — Signer Key         [🔒]
// Step 3 — Authorize Wallet   [🔒]

SetupTabPanel updated

• Replaces the generic SETUP_ONBOARDING_STEPS.map(...) card loop with <DownloadAntSeedStep />
• Buy, Manage, and History flows are untouched

QA coverage

• New DownloadAntSeedStep Storybook QA story (connected wallet, activeTab: 'setup')
• Two Playwright smoke tests: verifies step text ordering, href/target attributes on the download link, and tab structure isolation

• Fixes [PLAN] AI Credits onboarding: Download AntSeed step content #199

[Copilot]

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