Implement Sign up, Login, and Logout features

[elainefan331]

Description:
Implement complete authentication modal system with Sign Up and Login functionality. This includes frontend React components with Material-UI styling, form validation, and integration with the Express.js backend authentication API.

Tasks:

• Create UserSignup modal
• Create UserLogin modal
• Add logout feature
• Fix NavItems to render modals
• Connect to authentication endpoints
• Integrate login, signup, logout, get currentUser to redux
• Add google OAuth button
• Add orcid OAuth button
• Manual testing of flows
• Test error handling and persistence

Acceptance Criteria:
Login Modal

• Opens when "Sign In" is clicked in user menu
• Validates email format and required fields
• Shows/hides password with toggle button
• Displays error messages for failed authentication
• Closes and updates app state on successful login
• Has link to switch to signup modal
• Resets form when closed

Signup Modal

• Opens when "Create Account" is clicked in user menu
• Validates email format, password strength, required fields
• Confirms password matches with visual feedback
• Displays error messages for validation failures
• Prevents duplicate email registration
• Closes and updates app state on successful signup
• Has link to switch to login modal
• Resets form when closed

Authentication State

• User menu shows "Sign In" and "Create Account" when logged out
• User menu shows username, "Dashboard", "Settings", and "Logout" when logged in
• Authentication persists across page refreshes
• Logout clears authentication state and redirects to home