systemd user service should use user's default login shell

### Proposed change

The transient systemd unit should be configured to run with the user's default login shell from passwd.

### Alternative options

While it is possible to configure SystemdSpawner.default_shell, this setting affects all users on the system.
### Who would use this feature?

That's helpful for multi-user systems with users who have different login shells (bash,zsh, tcsh, etc.)

### (Optional): Suggest a solution

Locate the code that configures the transient systemd unit and inspect how SystemdSpawner.default_shell is currently applied. Check how the user's default login shell is represented in passwd, then verify that each user's unit uses that shell without changing the global setting; add or update the relevant tests if the repository provides them.

Written by the indexing model from the issue text.