app routes: add /signup, /login, /logout, and /profile routes

[cmnord]

Also make the header conditionally show the "log in" or "profile" buttons depending on whether the user is logged in.

[vercel[bot]]

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Name	Status	Preview	Comments	Updated (UTC)
jep	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 20, 2023 10:09pm