Static Asset Service UI Improvements

### Epic

_No response_

### Description

There's a delay from the moment you navigate to a new folder, the time it takes for the breadcrumbs to react to the folder change and the data to be fetched from the backend which is not aesthetically pleasing.

One solution would be to add a loader and fade out the files while the data is retrieved.

The Last modified column should be human readable.

### Which customers would this be available to

Everyone - CE/Starter/Team/Enterprise

### Acceptance Criteria

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate

No file, test, or entry point is named. Start by locating the Static Asset Service UI and its folder navigation, then trace the breadcrumb and backend-fetch loading flow. Done means the loading state is visually clear while files are retrieved and the Last modified column uses human-readable values.

Written by the indexing model from the issue text.