`Auth`: set up client-side authentication

[Seerden]

See #2.

Changes:

• implement client-side authentication
  • [x] add mutation hooks to log in and out
  • [x] add a reconciler hook that synchronizes client and server state on application mount
  • [x] add local storage helpers to retrieve, set and remove local user state

[Seerden]

Remaining tasks:

• Validation messages
  • [ ] redirect on success (but only from login page)
  • [ ] toast on failure (server, wrong password, nonexistent username)
  • [ ] set default 'me' state to localStorage user

[Seerden]

Happy path is functional. Work on remaining tasks later down the line, once the following are available:

• msw implementation -- so we can test react-query implementations
• workout overview or user page -- so we can redirect on successful login