chore: refactor frontend API client methods & add doc comments

[yusefnapora]

This refactors a few things related to the frontend login flow that I found awkward while reading through the code. I'm definitely open to suggestions / change requests if anyone thinks I've made things worse :)

The major changes:

• getToken from api.js has been renamed to getMagicUserToken, since there's a billion things called "token" in this codebase. Since this method only ever returns magic.link user id tokens, I gave it a more specific name, and also moved it to magic.js, since it's magic.link specific. Also renamed some of the global state vars related to saving the token to be more specific (e.g. LIFESPAN => MAGIC_USER_TOKEN_LIFESPAN_SEC).
• isLoggedIn got renamed to getMagicUserMetadata, since that's what it actually does, and a function called isSomething that doesn't return a boolean is surprising.
• added a logoutMagicSession function, so the logout component doesn't need direct access to the magic SDK instance
• made the getMagic accessor internal to magic.js
• added a getStorageClient accessor to api.js that returns an authenticated NFTStorage instance (using the key from getMagicUserToken).
  • this lets us stop exporting the API constant and the getToken functions, which were only used to new up client instances in UI code
• added type definitions for API response objects
• added doc comments

I also refactored almost all of the functions in api.js to remove copy/paste boilerplate

• there's now a fetchRoute helper that appends the route path to the API base url, sets the Content-Type to json, and unpacks the JSON response body
• fetchAuthenticated also sets the auth header to the value of getMagicUserToken

@redaphid would you mind checking this out and let me know if there's anything surprising?

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  Coverage   100.00%   100.00%           
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  Files            5         5           
  Lines         1242      1242           
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[hugomrdias]

LGTM just one console.log leftover