Add meta tags and structured data for SEO

Improve SEO with proper meta tags, Open Graph, and structured data.

**Acceptance Criteria:**
- Add dynamic meta titles and descriptions per page
- Add Open Graph tags for social sharing
- Add JSON-LD structured data for the landing page
- Add sitemap.xml generation

**Files:** `src/app/layout.tsx`, `src/app/page.tsx`, `src/app/groups/[id]/page.tsx`, `next.config.js`