feat: 멀티플레이어 네트워크 동기화 구현

- 로비 씬 추가 및 LobbyManager/LobbyUI/LobbySceneBuilder 구현
- NetworkPrefabsList로 플레이어 프리팹 등록 (PlayerPrefab 자동스폰 비활성화)
- PlayerMovement 서버 권한 이동 아키텍처로 전환
  - NetworkVariable<Vector2>로 클라이언트 입력 → 서버 전달
  - 점프 JumpRequestRpc로 서버 검증 후 실행
- 보스 프리팹에 NetworkTransform/NetworkAnimator 추가 (서버 권한)
- SkillController를 NetworkBehaviour로 전환
  - PlaySkillClipClientRpc로 클립 override + 재생 원자적 동기화
  - OnEffect/OnSkillEnd 클라이언트 실행 차단
- WeaponEquipment 클라이언트 무기 시각화 동기화 수정
  - registeredWeapons 인덱스 기반 NetworkVariable 동기화
  - SpawnWeaponVisualsLocal로 클라이언트 무기 생성
  - 중복 Instantiate 버그 수정

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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