Update Cozystack screenshot (#1459)

ref https://github.com/cozystack/website/pull/335

Signed-off-by: Andrei Kvapil <kvapss@gmail.com>

<!-- Thank you for making a contribution! Here are some tips for you:
- Start the PR title with the [label] of Cozystack component:
- For system components: [platform], [system], [linstor], [cilium],
[kube-ovn], [dashboard], [cluster-api], etc.
- For managed apps: [apps], [tenant], [kubernetes], [postgres],
[virtual-machine] etc.
- For development and maintenance: [tests], [ci], [docs], [maintenance].
- If it's a work in progress, consider creating this PR as a draft.
- Don't hesistate to ask for opinion and review in the community chats,
even if it's still a draft.
- Add the label `backport` if it's a bugfix that needs to be backported
to a previous version.
-->

## What this PR does


### Release note

<!--  Write a release note:
- Explain what has changed internally and for users.
- Start with the same [label] as in the PR title
- Follow the guidelines at
https://github.com/kubernetes/community/blob/master/contributors/guide/release-notes.md.
-->

```release-note
[]
```

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Updated README screenshot to use the dark-theme image, improving
visual consistency for viewers using dark mode.
* Purely presentational change — no impact on application behavior or
public interfaces.
* Clarifies repository appearance for users browsing in different
themes.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

README.md

@@ -19,7 +19,7 @@ Database-as-a-Service, virtual machines, load balancers, HTTP caching services,
 
 Use Cozystack to build your own cloud or provide a cost-effective development environment.  
 
-![Cozystack user interface](https://cozystack.io/img/screenshot.png)
+![Cozystack user interface](https://cozystack.io/img/screenshot-dark.png)
 
 ## Use-Cases