Integrate the auth system on frontend with the backend

[joulev]

• Update user display name (PUT /users/:uid)
• Update user photo (awaiting Routes to upload images #98)
• Delete user (DELETE /users/:uid)
• Handle errors (awaiting How does the backend report errors? #101)

All other actions require authentication by popups so are really hard to do with the backend, so we will use client-based Firebase for that.

Until this PR is merged, the deployment can be viewed at https://test.joulev.dev.

[codecov]

Codecov Report

Merging #102 (969059b) into main (c477c5b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #102   +/-   ##
=======================================
  Coverage   99.22%   99.22%           
=======================================
  Files          29       29           
  Lines         389      389           
  Branches       94       94           
=======================================
  Hits          386      386           
  Misses          3        3           

[joulev]

Not much to really review, everything looks good to go.