Add mock AssignmentsApi

[katariniss]

I as a developer
Want to add mock AssignmentsApi in code
So that to get response from backend until Project Module is not implemented

Description
Create a mock AssignmentsApi for projects available to the logged-in user for a period by date:
GiveMeProjectsForPeriod(startDate, endDate)
It will return not just an array of projects, but an array of projects for each date in the period:
{
"2025-12-17": [...],
"2025-12-18": [{id: 1, name: "Inner Circle"}]
}
The main point is that today you can be assigned to a project, and tomorrow you won't.

Definition of Done
For example:

1. Frontend gets a list of projects for a period by date

[katariniss]