feat: auth context setup

[carolynzhuang]

What's new in this PR

Description

created auth context for login

• if isAuthenticated = true, routes to tree search screen
• if false, routes to login screen
• placeholder logout button in contact screen, routes back to login after logout completed

Screenshots

How to review

1. log in, should lead you to tree search screen.
2. restart the app, should automatically start on tree search screen
3. go to contact screen and log out. make sure you are directed back to the log out screen.
4. restart app and make sure that it starts on login screen

Next steps

style logout button

Relevant links

Online sources

Related PRs

CC: @christophertorres1