[nav] Implement expo routing and add global styles

[akshaynthakur]

What's new in this PR

• Refactor nav using expo routing instead of react nav
• Add blanks screens in auth flow (signup, onboarding)
• Add global styles sheet with temporary styles

Relevant Links

Notion Sprint Task

Online sources

SUPER helpful video (better than written docs imo): https://www.youtube.com/watch?v=4-shpLyYBLc

Related PRs

How to review

• Click all buttons and bottom tabs, ensure that routes are set properly and pages render

Next steps

• Screen-specific styles: adding one styles sheet per screen inside of a folder seems tedious and complicates an already complex file structure
• Need to prevent navigating back to prior screen in certain cases, eg. going back to auth/login once on the home page. Tried using router.replace() but induced weird behavior because didn't fully clear the stack -> could still go back to the -2 page on the stack

Tests Performed, Edge Cases

• Tested on both android and ios to make sure SafeAreaView working properly

Screenshots

CC: @akshaynthakur