Save (unauthenticated) time slot selection on log in

Hey newdlers! 🍜 I was just hit by this and thought it might be a nice UX enhancement for the project:

### Steps to reproduce:

1. Open a newdle on incognito mode.
2. Fill in your availability.
3. Realize you're not authenticated and hit the "Log in" button.
4. Login through your IDP.

### Expected result
The newdle no longer asks me _Who are you?_ and allows me to send my answer already.

### Actual result
I'm logged in, but my availability selection has been cleared.

Start by reproducing the flow in an incognito window: select availability in a newdle, choose “Log in,” and complete the IDP login. Trace how the selection is handled across authentication; done means the logged-in user sees the existing availability and can submit it without reselecting.

Written by the indexing model from the issue text.