Include project name in task assignment email notifications

Hi,

When a user is assigned to a task, the email notification shows the task title but not the project name. For teams working with multiple projects, task titles can be similar or generic, so the recipient cannot identify the context without opening Vikunja.

Expected behavior

Please include the project title in the assignment email body, for example:

Alex has assigned you to “Clean up”.

Project: “IT Operations”

The email subject can remain unchanged.

Technical context

This appears to be a small presentation-only enhancement; no additional database lookup should be needed.

It would make assignment emails much easier to triage without opening each task.