Move intel compute runtime to dedicated component

### Bug description

See: https://github.com/canonical/gemma3-snap/pull/50#discussion_r2727442462

If we move the compute runtime to a component, we need to make sure this component gets downloaded when the snap is refreshed. We can do this in a post-refresh-hook.

1. Check if the currently active engine still exists. If not, auto select one.
2. Check that all components for the currently selected engine is installed, otherwise install them.

### To reproduce

n/a

### Snap version

```shell

```

### System information

_No response_

Start with the post-refresh-hook behavior described in the issue and review the linked pull request discussion for the component-download context. Confirm the active engine remains available after a refresh, select one when it does not, and ensure all components for the selected engine are installed.

Written by the indexing model from the issue text.