refactor: preview modal now renders the real threejs template sections

Replace the custom preview design with the actual HeroSection,
GitHubSection and StatsSection components from the threejs template.
Live GitHub API data is converted to the exact prop shapes those
components expect:

- buildHero   → hero + snapshot props for HeroSection
- buildRepos  → ProjectRepo[] for GitHubSection
- buildStats  → Stats object (metrics, languageDistribution,
                activityByYear, topReposByStars) for StatsSection

The user sees the real template — same fonts, layout, 3D charts,
language pie, bar charts — before deciding to fork.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: amide-init