Implement group member profile cards

Show profile information when clicking on a member in the member list.

**Acceptance Criteria:**
- Create MemberProfile popover/modal component
- Show: address (shortened), join date, contribution history in this group
- Display reputation score if available
- Add link to view full address on explorer

**Files:** new file `src/components/MemberProfile.tsx`, `src/components/MemberList.tsx`