Validate Niri config after `dms sync`

### Feature Description

Run `niri validate --config /etc/greetd/niri/config.kdl` after `dms sync` and print out the results.

### Use Case

I have
```
debug {
wait-for-frame-completion-before-queueing
}
```
in the user config.

When `dms sync` is used, `/etc/greetd/niri/dms.kdl` has
```
debug {
honor-xdg-activation-with-invalid-serial
}
debug {
wait-for-frame-completion-before-queueing
}
```

This results in Niri failing to start after login.
By validating the config, issues like this can be caught.

### Compositor(s)

Niri

### If Other, please specify

_No response_

### Proposed Solution

_No response_

### Alternatives/Existing Solutions

_No response_

Start by locating the implementation of `dms sync` and the compositor configuration sync path. Run `niri validate --config /etc/greetd/niri/config.kdl` after syncing and inspect how its results are reported. Done means validation runs after `dms sync` and prints the results so invalid Niri configurations are visible.

Written by the indexing model from the issue text.