[AN][refactor]: 로그인 화면 컴포즈 마이그레이션

[junseo511]

🤷 구현할 기능

💬 구현 배경

🔨 상세 작업 내용

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📄 참고 사항 (Optional)

[coderabbitai]

A summary of the changes CodeRabbit can apply:

• Migrate the login screen from XML/ViewBinding + LiveData to Jetpack Compose + StateFlow by creating android/app/src/main/java/com/mulkkam/ui/login/LoginScreen.kt, updating android/app/src/main/java/com/mulkkam/ui/login/LoginActivity.kt (BindingActivity → ComponentActivity, setContent with LoginScreen, remove ViewBinding) and android/app/src/main/java/com/mulkkam/ui/login/LoginViewModel.kt (LiveData → StateFlow, expose authUiState and isAppOutdated), and plan to remove android/app/src/main/res/layout/activity_login.xml as a follow-up while preserving existing Kakao login, back‑press, update dialog, navigation and tests.

• Migrate the login screen from XML/ViewBinding to Jetpack Compose by adding COMMIT_MESSAGE_TEMPLATE.md and MIGRATION_GUIDE_LOGIN_COMPOSE.md, creating a new LoginScreen.kt with composables (LoginScreen, LoginContent, KakaoLoginButton), converting LoginActivity from BindingActivity to ComponentActivity with setContent that hosts the Compose screen, replacing LiveData/SingleLiveData in LoginViewModel with MutableStateFlow/StateFlow (including isAppOutdated), removing view-based observers and XML-dependent code, and preserving existing behaviors (Kakao login, navigation, update dialog, double-back exit) while adjusting snackbar/back-press handling to use root view.

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