[PROFILE-005] Reuse profile card for onboarding preview

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Outline

Summary

Replaces the bespoke onboarding preview markup with the shared discovery ProfileCard (variant="preview"), so the onboarding card now matches the profile editor preview and the discovery card, and stops drifting from them as the card evolves.

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Changes & Enhancements

• Type of Change:
  • Feature
  • Bugfix
  • Refactor
  • Chore
• Changes:
  • (onboarding):
    • Render the onboarding live preview with ProfileCard variant="preview" instead of duplicate card markup.
    • Map onboarding draft values into a DiscoveryProfile, mirroring the profile editor preview, and convert the availability enum with availabilityPresetToPattern.
  • (i18n):
    • Add previewPrimaryFallback and previewNoTags Onboarding keys (en, ja).
    • Remove the now-unused previewPublic and previewAvailabilityFallback Onboarding keys.

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Page / App Changes

Affected Next.js Routes (Local):

• /[lang]/onboarding

Before (Screenshot)

After (Screenshot)

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Component / Storybook Changes

Affected Storybook Components (Local):

Before (Screenshot)

After (Screenshot)

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Not Doing

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Other (Remarks)

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Checklists

• No out-of-scope changes are included.
• Removed unnecessary debug code (e.g., console.log, debugger).
• Self-reviewed and tested locally.