Improve test coverage

[raimohanska]

There are TODOs in at least

• Playwright tests
• board-store.test.ts

Also, Playwright tests with authenticated users, creating private boards. Test that re-joining a private board works (was broken), test the cleaning of locally stored private boards on logout (was broken). This requires a mock authentication endpoint for testing. For instance, create an endpoint /auth/mock/{user} that will authenticate you as {user}@ourboard.io.

Good tests keep Ourboard stable when boldly developing new stuff. Writing Playwright tests is good fun!