fix(fe): revert signup button to modal

[ojongii]

Description

Sign Up 버튼 클릭 시 /signup 페이지로 이동하는 대신 기존 방식대로 모달이 열리도록 변경했습니다. 새 회원가입 페이지는 라우트를 변경하는 대신 /signup 경로를 그대로 유지했어요.

Additional context

closes TAS-2683

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[gemini-code-assist]

Code Review

This pull request refactors the HeaderAuthPanel component to consolidate authentication triggers within a single Dialog component. I have identified three areas for improvement: the Dialog's onOpenChange handler needs to be updated to prevent conflicts with trigger clicks, the DialogTitle requires a descriptive label and should use sr-only for accessibility instead of hidden, and the visibility change for the Sign Up button on mobile devices should be reconsidered to ensure authentication options remain accessible.

[skkuding-bot]

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[dayeoni]

네! 고생하셨습니다~~👍

[skkuding-bot]

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