feat: onboarding flow — post-OAuth GitHub data connection + score generation trigger

[basanth-p]

Summary

Build the onboarding flow shown after first GitHub OAuth login. Replaces the placeholder in app/(dashboard)/dashboard/onboarding/page.tsx.

Steps in the flow

1. Welcome screen — "Let’s build your Patch ID" with GitHub avatar + username
2. GitHub data pull — show repos, PRs, contributions being fetched
3. Score generation — loading state while Core engine computes first score
4. Reveal — animated score card reveal with breakdown
5. Share prompt — copy shareable profile link

Acceptance criteria

• Only shown to users where isNewUser === true from auth callback
• Subsequent visits go straight to /dashboard
• Handles GitHub API rate limits gracefully
• Mobile responsive