feat: implement Contributor Passport, public profiles, and privacy controls (Phase 2)

[ashishsinghbora]

What changed

• Built public Contributor Passport pages at /people/<username> displaying real GitHub-verified proof of work, statistics, credentials, and chronological contribution timeline.
• Added dynamic Open Graph social card generation at /people/<username>/opengraph-image using next/og.
• Added public Contributor Directory at /people listing verified contributors with level badges and founding numbers.
• Implemented Contributor Settings at /dashboard/settings with public/private privacy toggle (isPublic), bio, portfolio/LinkedIn links, and skills editor.
• Added comprehensive unit and integration tests covering privacy rules, statistics aggregation, and timeline generation in Vitest.

Verification

• Vitest: 7/7 suites, 22/22 tests passing.
• TypeScript tsc --noEmit: 0 errors.
• ESLint: 0 errors, 0 warnings.
• Production build: all 12 routes compiled and optimized cleanly.