Backend:
- Add MembershipResponse and PendingMembershipResponse DTOs
- Add GET /memberships/me and GET /admin/memberships/pending endpoints
- Add findByUserIdWithDetails and findPendingWithDetails queries
- Add getOwnMemberships and getPendingMemberships service methods
Frontend:
- Add membership service for API calls
- Add join-community page for users to browse and join communities
- Add my-memberships page for users to view their memberships
- Add admin-memberships page for admins to approve/reject requests
- Add routes and navigation items
Tests:
- Add MembershipServiceTest with 13 unit tests
- All 20 tests passing (UserServiceTest + MembershipServiceTest)
