`Users`: register client-side

[Seerden]

Relates to #1.

Changes

• [x] create Register component with change, blur, submit handlers to build a newUser object
  • [x] test form and render functionality
• [x] implement a fetch function and query hook to POST to /user/register and store the inserted new user in the query cache

[Seerden]

Follow-up:

• [ ] log in automatically client-side (and server-side) on successful mutation (see e.g. #2)
• extend Register form:
  • [ ] add button to toggle password visibility
  • [ ] implement better feedback messages as toasts
• extend user account:
  • [ ] optionally implement registration by using email
• [ ] figure out the best way to test query and mutation hooks (MSW?)