Add loading indicator when clicking "Show on Map" in Assets tab

[saidy-moregeo]

When clicking "Show on Map" for an asset in the Assets tab, there is no loading indicator while the map is being prepared. This creates a poor user experience as users may think nothing is happening, especially when the map takes several seconds to appear.

Steps to Reproduce:

1. Load the following catalog source: https://sitn.ne.ch/services/stac/ne/lidar2022/collection.json
2. Navigate to the Assets tab
3. Open the "bounding box" asset
4. Click "Show on Map"
5. Observe that there is no loading indicator while waiting for the map to render

Expected Behavior:
A loading spinner or indicator should be displayed after clicking "Show on Map" to provide feedback that the action is being processed.

Actual Behavior:
The UI remains static for several seconds before the map suddenly appears, giving no indication that anything is happening.

[m-mohr]

I checked this today, but this is going through so many components with events that we it's impossible to feed back the state in the current architecture.

The flow currently is:
HrefActions -> show event -> AssetAlternative -> show event -> Asset -> showAsset event -> Catalog / Item -> sets selectedAsset -> ShowAssetLinkMixin -> reactivity -> Map -> watchers -> ol-stac

We'd need to create a connection between HrefActions and the watchers in the Map component, i.e. we'd need to reintroduce the EventBus again, I think.