Add skeleton loading for GroupCard component

Create a skeleton version of GroupCard for loading states.

**Acceptance Criteria:**
- Create GroupCardSkeleton matching GroupCard layout
- Animate with shimmer/pulse effect
- Use in groups list page while data loads
- Show 6 skeleton cards during initial load

**Files:** new file `src/components/GroupCardSkeleton.tsx`, `src/app/groups/page.tsx`