feat: 회원가입 페이지에 대한 디자인 시스템 적용

[MTtankkeo]

작업 내용

• 회원가입 화면 전반에 GDS 디자인 시스템 컴포넌트를 적용
• 닉네임 입력, 약관 동의, URL 입력 단계를 PageView 기반 흐름으로 정리
• 기존 개별 버튼들을 하단 공통 CTA(SignUpButton)로 통합
• 회원가입 완료 후 환영(Welcome) 화면을 추가하고, 시작하기 버튼으로 홈 화면에 진입하도록 변경
• 닉네임/URL 검증 성공 상태를 입력 필드에 표시
• 회원가입 성공 시 UserAuthProvider의 사용자 정보를 갱신하도록 반영

피그마 참고:

모바일: https://www.figma.com/design/BA5LeHKF3saevi3poE85n2/Grimity_APP?node-id=1034-84144&m=dev
테블릿: https://www.figma.com/design/BA5LeHKF3saevi3poE85n2/Grimity_APP?node-id=1001-268806&m=dev

[gemini-code-assist]

Code Review

This pull request refactors the sign-up flow to integrate the new Design System (gds), introducing a welcome state and replacing old custom widgets with GDS components. The navigation is updated to use a PageView with a PageController, and the sign-up state management has been streamlined. The review feedback highlights a bug where error: true is hardcoded in SignUpNicknameTextField, and suggests optimizing SignUpButton by removing redundant ref.watch calls and simplifying the widget tree by passing onPressed directly to the button.

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